Has anyone put S4 springs on their A4 2.8? I have a 99 2.8 quattro and was wondering how much it lowers the car. My car is the non-sport model also.

I say go with the an aftermarket shock/spring setup. The S4 setup isn't anything to brag about and its not gonna make the car much lower.

you need coilovers and the h-sport sway bar set. you will increase the smile factor of driving your a4 by tenfold. i promise.

I have them on my 98.5 a4.
Yes they will fit and work well due to the fact that the 2.8 and S4 have similar front end weight.
You'll get a stiffer (but comfortable) ride. you'll want to swap out the shocks too, as stock shocks will blow out after a while
You won't get verylow with them . I believe you'll drop about a quarter inch over stock. Where I live (No. NJ) that's fine.
If you decide to go with these springs and shocks, put $$ away for new control arms an tie rods. The firmer ride will make them wear out faster.