I have to disagree with the assumption that the motor is not an issue. Mark 'Speedwell' Bennet pointed out, there are a thousand 116HP 1.6L's out there, but probably very few 122HP+ and even less 125HP+ 1.6L's out there. A 116HP 1.6L is not going to compete with a '99, but a 125HP 1.6L will. I'm in the 116HPish camp, even though I have a pro built motor. I'll admit I probably haven't spent enough time on the dyno, but will I get there with just AFM and fuel pressure tuning?
Mark mentions exhaust tuning (needing a good exhaust manifold and downpipe), but for most of us, showing up to a dyno tuning session and swapping manifolds is a pain in the ass. I suppose you could buy a bunch and throw them on a flow bench, or is it how well they match the head?
So how much are the external bits worth (exhaust manifold, downpipe, air filter, etc.) and how much is the AFM tuning worth. Can I get into the 120's with my current engine by doing a better tuning job?
So back to the discussion of AFM tuning. So, let's forget the A/F ratio reading and just go by torque and power. Is there a sweet spot? If I go too lean, will I start to lose power and torque? I assume so, but does this happen at a point when I'm already too lean as far as melting down my engine is concerned. I understand the spark map. If I loosen the clock spring, and correct A/F ratio using fuel pressure, the AFM will think the engine is at a higher load than it actually is, giving me a flatter spark map (less advance at higher rpm). It would seem then, using both the sweeper adjustment in the AFM and using an adjustable fuel pressure regulater would allow me to independently adjust where I am on the load/spark advance map and were my A/F ratio is.
I'm sure nobody wants to give away trade secrets, but I assume the best AFM tuning is to loosen the flapper (clockspring) and adjust the sweeper back to counter the fact that the flapper will now be open wider for a given airflow. Adjusting the sweeper back gets you back to the point on the spark advance map you want to be on, and leans the car back out. Probably gets you close, but the adjustable fuel pressure can then fine tune your A/F ratio. Rotating the AFM so gravity helps keep the flapper open helps reduce restriction even further and keeps cornering forces from screwing with the A/F ratio. Am I close???? Setting base timing does not accomplish the same, since I assume it just shifts the entire map up and down?
I didn't adjust the sweeper during my dyno tuning session last year since I assumed it accomplished the same thing as my fuel pressure regulater, in that it just adjusted the A/F ratio. Mark's comment on the spark map made me realize that it is not the same. I will be scheduling another dyno appointment to try out my above theory.
I truly believe that if we could get more 1.6's running in the 122HP+ range, that parity to the '99s would not be an issue.
Marc
Somewhat self-serving post: I spent years and seriously ~1000 dyno runs between 2 engineers, 2 cars, 5 engines, 2 engine builders, with a local dyno charging us almost nothing, figuring out the AFM. You are better off starting with one already "tuned/adjusted", and use your AFPR or the clockspring to finetune. Buy it from me, or Drago, or Stewart, or RE, or anyone else that knows what they are doing - but it is NOT worth trying to reinvent the wheel yourself. I'm not leaving people off of that list because of politics, I simply know that the aforementioned "know what they are doing" w.r.t the AFM. I'm sure there are others that know what they are doing, but I also know there are some AFM tuners lost in the weeds. And I also firmly believe that an AFPR alone will NOT get you best T and HP because it does not and cannot address the restrictor plate and spark map issues. An AFPR WILL get you perfect AFR, but there is no trophy for perfect AFR 
The adjustments are extremely "iterative", and the wiper adjustment is particularly sensitive. Take a video of an opened up AFM at WOT on the dyno, and you'll see the wiper barely moves in the region we care about (WOT >5700 RPM).
I see "tuned" AFMs that are awful, and I see ones that are as good as I would do myself. The 1.6 is so nonrepeatable on the dyno, and since "flat" AFR generally AIN'T the right answer, you are PROBABLY BS-ing yourself if you are trying to do it yourself - UNLESS you are insane like we were and running 2 cars 25+ runs in a session, several times a year.
If I had a nickel for every 1.6 guy that said: I went to the dyno and made 120 (or 116), but last time I made 116 (120)! .... I'd have at least $115. It takes religious dedication, a well-maintained and patient dyno vendor, and 100s of runs, to separate "noise" from "signal" w.r.t. 1.6 dyno testing. If your laptimes and straight-line data are good, that is probably MORE reliable than the dyno.
I have never seen a 1.6 make best T and HP when it is "too lean". If your dyno days are convincing you that "too lean is better", check the 02 calibration and spark plugs. >13:1 is not "too lean" for a 1.6. >14:1, be concerned .... but if plugs look OK, no audible sounds of knock, and you are running good fuel, you are probably OK.
Tip: I always found Sunoco GT100 (aka CAM2 Unleaded 100) was worth 1.5/1.5 on the 1.6 .... and worth nothing on the 99. I ran GT100 always because I could trust it, I could wash my car with it, I could wash my face with it, and it was $5.60/gal from my local Speedway. It is gasoline as God intended it in 1967. Clear, dries with no residue or odor. Good stuff.
When it comes to DPs and headers, YES, flow them. No, we aren't "matching" anything -we are just looking for best-flowing parts and in the end you'll tune to match that stuff. Most of our namebrand known SM engine builders know what a good header is, and can flow yours, and probably provide you a "good" one. OR, ask your engine builder to provide you a good one. Chances are he is working with a vendor that is "recreating" the best header Mazda ever made. Decide for yourself whether you will lose sleep over that. "Bad" headers tend to show up as a "knee" in the HP curve at high RPMs - again, acting like a restrictor plate. Bad headers always look bad inside, but sometimes headers that LOOK bad are surprisingly awesome. SOP is to acquire many, send to your engine builder, have them flowed, pick the best. Early 1990 headers tend to be "gold", but they also tend to be already leaking or rewelded, and you MUST MUST MUST run the damn horseshoe brace to prevent future leaks and failure.
+1 for what Fowler said. It takes only a SMIDGE of brake drag or crappy alignment to piss away 2-6 HP.
I'm starting to get obsolete on where parity is, but my gut says the 1.6 is a good horse for a clever guy with fast hands! Outqualify those 99 "tractors" and check out before they can get in front of you and kill your midcorner advantage
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