Okay, I realize this isn't real world but let me tell you, I believe it's damn accurate... Anyways...

I just got Forza Motorsport for Xbox last night... It has a 2000 S4 and a 2004 S4... The 2000 sounds so damn accurate to the real thing. I've never driven a 2k4 so I wouldn't know.

Anyways, I took it to an oval test track and tested its high speed... It gracefully pulled to 157 MPH... From that point, after I cleared the bend, it slowly but surely got its way up to 161 MPH.

Now, to see if t his was accurate or not, I decided to take a car that I did know the stock top speed numbers of, in 5th and 6th gear...

I grabbed a hold of a 2004 Celica GT-S (I owned a 2000 at one point) and took it to the same track. In 5th I managed 141 MPH, and in 6th it dropped to 139, and then a while later it reached 140 but would drop back to 139 with the slightless movement... This was simply amazing because all magazine reviews when the GT-S first came out quoted the car would not go over 139 in 6th but could pull a couple extra MPH if you took it past its redline in 5th!

So, for the original question, the B5 S4 should easily reach 160 MPH completely stock (minus the governor).