"Strength" I believe may not be an accurate term. I think over time certain minor aspects were changed at the original source to improve functionality and eliminate certain things from happening(reverse at speed?). But I do not think the actual gears and main shaft have changed? And when a "rebuilder" has the unit apart they are changing wear items...Syncros, syncro springs, shift forks, roll pins, all seals, etc. All gears and probably shift hubs remain? The time and expense to replace all isn't practical.
All that said...I think we are definitely beginning to reach life expectancy of 3rd gear which seems to blow up/fail at the highest rate. One of our local guys will replace 3rd gear for an additional rebuild cost but apparently Mazda is not able to keep stock of these. He said he bought every piece Mazda had awhile back and hasn't been able to get more(this was a conversation 2 months ago). I had 10 junkyard trannies 1 year ago and have none now. Getting harder to find easily. I wont hesitate to use either NA or NB. In the end It is a crap shoot on its life expectancy...and if you are a shitty shifter and running a Unsprung clutch disc good luck. Jim has always advocated a stock PP as well I believe. Eliminating the "shock" on these units is always a good thing!
I will gladly be corrected if anything here is not accurate...always learning!
For those trying to diagnose problems...if you suddenly begin missing shifts and/or grinding...check the easy things 1st! Pull the shifter and make sure the plastic bushing on the ball hasn't broken...also make sure your clutch slave cylinder has a good throw...barely disengaging the clutch will make it harder to shift(maybe air in line?)