There are three types of motors.
The SOHC which was standard in the base and on some ES
The DOHC which came with most ESs and standard on the RT
The TT DOHC which only came on the RT/TT
The DOHC has around 10.5 compression where the TT version only has 8.8 or so. I have seen guys with RT/ES models put a single turbo on there cars for extra power. The RT/TT model cqame with a 5 speed for three years until switching to a 6 speed, this switch came the same year as the headlights went from flip-ups to projector beams.
Hope this helps somewhat.
for more info check this site out....
http://www.3si.org/portal/index.php?s=
that site is by far the best site I have found for info for my car