I have found a pair of 9 inch rear wheels for sale, at a reasonable price. I currently have the stock wheels on my car (being '84 shark with US spec engine and 4 speed auto) which are 7 inch wheels all round. I see across several road tests that:
1) early cars had 7 inchess all round
2) S4 series had 7's in front and 8's in rear
3) GTS had 7.5s in front and 9s in rear

I know later series 944 Turbo had 7's in front and 9's in rear.

Has anyone had 7s in front and 9s in rear on a 928? Will it improve the handling much, or induce too much under-steer? All thoughts appreciated.

 

If they fit it shouldn't cause much if any trouble on the street. Like you mention, later 928s had staggered sizes. I would think you could run 225 section fronts and 255 section rears but suspect the main improvement would be in looks not performance.

 

I have 7" cup 1's and 9" cup rears with 225/45 and 255/40 tires respectively. I drove it yesterday down to a buddies shop and took a pretty twisty road for a short distance. I took some 30mph corners at about 60 and it felt pretty good. I dont think you'll have trouble with the car on the street, on the track you'll probably get some understeer with that setup.

 

It all depends on the wheels offset and bolt spacing...our lugs are 5x130mm & the offsets vary by wheel-MY

It should work unless the offsets are really off.

 

Good point, for a frame of reference my wheels (both front and rear) have 55mm offsets.