General

Differences between S4 and GT

Here are the physical differences between an 89S4 5-speed and 89 GT:

89GT has more precise pistons(smaller well between valve pockets for exact 10:1 compression)
89GT has high lift and longer duration cams
89GT lighter and more free flowing exhaust
89GT has different engine computer mapping
89GT idles at 775 vs S4 675
89GT has rev limit at 6800 vs S4 6600rpm
89GT torque peak at 4100 vs S4 3200
89GT 326 hp vs S4 316hp
89GT 8" and 9" wheels vs S4 7" and 8"
89GT 17mm rear bolt-on spacers for wider track
89GT has Factory "cringed" fender lips to guarantee clearance with wider track.
89GT has 20mm shorter shift lever. (10% less travel)
89GT has standard Limited Slip (40%) and standard Sport Shock/Spring Option
89GT has slightly different Instruments
89GT has "Straight Cut" racing type ring and pinion.

I have both and the S4 has slightly more torque on the low end for normal derivability, however once that GT hits 4000rpm it is a "Freight Train" and with the close-ratio 5-speed all of the upshifts hit right in that torque peak (between 4100-4300). The GT likes to be driven hard and gives many rewards in doing so.
The 89 GT is also a much stiffer ride than the S4 ;-) The 89GT is also much rarer than the S4 and therefore worth more. It was kind of a prototype for the 90-91 transition of all 5-speeds being a GT and all Autos being an S4.

When you move up into the 90 GT there were slight differences, but have the same engine etc. Here is what was lost and added on the later normal 90-91 GT :

8" and 9" Club Sport forged wheels replaced with 7.5" and 9" cast Design 90 wheels.
Deletion of 17mm bolt on rear spacers.
Fenders lips were not cringed because of the narrower track
Normal cut ring and pinion gears
40% fixed LSD replaced with computer controlled variable PSD
Dual Airbags added.
Oil cooler added below the lower radiator support along with extra air slot in front spoiler.