As far as I know... the engines are the same... HOWEVER... there were 3 different power rating 1.8 Ztecs...

I belive one was 105 another was 115 and the thrid was 120

so not much... not sure on the why though..

also just incase you didn't know...

Fairly straight forward to turn your SI into a GTI spec wise as the wires for the mirrors are there. Sun roof electric window I think isn't but again, is dooable. as for body kit... 10p a dozen i think