what i found in my experience in driving a slightly modified gts-t and a stock gtr (which i bought stock) both 33s:
1) the gtr is laggier
2) the gtr likes to understeer more
3) the gtr feels much more torquey
4) the gtr has much longer legs than the gts-t
5) the gtr's power delivery is much more linear than the gts-t's
i spent only a year driving my gts-t, but that was my first turbo car. i think the attessa is an awesome thing to have in a car (if you know how to make the full use of it), but i won't say it's idiot-proof.
so in my opinion, a modified track gts-t and a stock gtr on the track, i doubt the gtr will have too much problems keeping ahead (all things being equal, i.e. driver skill). but this all comes down to corner exit, with the gtr being able to get into corners harder and not get sideways too much, or unnecessarily, and that the gtr will be on WOT earlier than the gts-t making for a higher exit speed and subsequently faster down the straight.
for dragging however, i think with 26k on the gts-t would see it outrun a gtr over the quarter, simply based on the fact that the richer man will have the faster car.
and for drifting, i somehow don't think that the gtr is the most suitable of cars.
for a street driven car, i'm gonna say it's hard to drive a 10 sec gts-t as opposed to a stock gtr on the streets for obvious reasons.