Actually, there are many "performance" parts for manual transmissions. Parts like better input / output / main / countershafts, bearings where bushings are normally used, stronger cases, stronger or non-stock ratio gears, and shift forks are available for different transmissions. But us FWD guys rarely get to see any since there's just no market for it.
The first thing I do for my own trans is to replace the synchros, whether performance or stock rebuild. I'm not sure for the Getrag / Muncie / NVG trans, but if your unit comes with paper lined synchros and you can replace them with the forged steel type it's probably a good idea to do so. The condition of the teeth on the driven gear which mates with the sunchro sleeve is extremely important. Badly beaten teeth can cause difficulty in shifting. And the shift forks cannot be worn unevenly or broken or else they will need to be replaced.
Look for bushings worn excessivly as evidenced by outer coatings being worn through of by excess play around a shaft or between two parts. You can "blueprint" a trans in the same way you blueprint an engine, by putting in extra work to make sure all parts and dimensions are at the optimum spec.