Kevin - brake pressure is a very useful tool when it comes to analyzing braking and trailbraking. It's one thing to look at velocity, but brake pressure will give very detailed insight into what's happening. Most of my clients already have brake pressure or we add it - AIM and Traqmate users alike.
Steering wheel position can be helpful too, but I usually watch the video to get a sense of steering angles. This is a bit more of a luxury as far as sensors go.
Downshifting - I get this question a lot, so let me give several answers. First, downshifting is just a means of being in the right rpm range. If you are downshifting 3 gears, 1 gear, or not at all, the approach doesn't change. In other words, the downshifting should not affect your braking or turning. Downshifting happens in the background - kind of like a Windows update but without the reboot afterwards. With enough practice, it becomes second nature (right about now you should be thinking "muscle memory").
As you eluded to, it's pretty easy to overslow a corner trying to get a downshift in. If you suspect that is a problem you can try skipping the downshift(s) one corner when you are running alone and then compare entry speeds to a lap where you downshift.
The next answer is this: it doesn't matter if you go 5-4-3-2 or 5-2 in terms of lap times. If you feel overwhelmed or rushed, it may help to just do the 5-2 but of course you need to do this very late in the brake zone or you'll be buying a new motor. And you need to be pretty good at matching revs.
I personally go through all the gears on downshifts as I don't like the revs to drop too low and I'm always in the right gear for any particular moment in time. It also gives me a good sense of when I can make the final downshift since the revs are still in the right range (also a helpful tool for learning what gear to be in on new tracks).
Full disclosure: SMAC chairman, my opinions do not reflect anything to do with the SMAC unless specifically stated.
Todd Lamb
Atlanta Speedwerks
www.atlspeedwerks.com
SpeedShift Transmissions - reliability and performance
Spec Miata / Spec Boxster / Spec Cayman specialist
Spec MX-5 Challenge Series Director
Global MX-5 Cup team