Just a though, as I understand it the crankcase of a GT3 is based upon the 964/993, so in theory would a GT3 6 speed fit a 964, I know there is some messing around to make the speedo work
The rev drop 1st to 2nd on the 6 speed is actually bigger than the 5spd. 6th gear give less than 2mph/1000 revs. There are better ways to spend money imho.