input gets abused on the starting line
anyone that had a T5 back in the day knows the input is not always what breaks
in the T5 it was deflection between the main shaft and counter shaft that would move the tooth contact point to the tips of 3rd gear and rip teeth off
I think the story on th tr3650 is that people swap them out and sell them to put in a better transmission
which is how I got mine on ebay
the list of internal improvements mentioned in this thread is still pretty short, input shaft.
syncros and shift forks are messed up by the driver and cannot be considered a strength upgrade.
to reduce shift fork damage we have shifter stop bolts, driver skill is the only thing to help syncros (learn to drive/match rpm)
the best mod for drag manual trans is to remove the syncros altogether