I think part of it has to do with engrams and muscle memory. The muscle movements of clawing your way up Phoenix are a world apart from the chops in Karate Crack. Doing one is not going to help much in the other, apart from some body tension stuff. Even sport climbs (as I found out on Heresy today) vary widely in what muscle groups and movements are used. There are a lot of gym boulderers who can waltz up overhanging jug routes but flail desperately on the thin stuff at Smith. Crack is even more foreign to them, so I would expect it to be the same for a crack specialist climbing face stuff at Smith.
I'll lead some 10a/b sport stuff, the odd 5.9 trad crack. These ratings can fluctuate though. As far as top rope vs. leading, I find I'm sometimes climbing worse on TR because I psych myself out by thinking "don't fall here you pussy toproper, your partner just led this ." As you can see I am not self-concious at all