...I got 173 out of the S5 on a private road before I ran out of space and it was still increasing, so probably top at about 180mph.
...as for the TDi, the problem is that the torque drops away a lot at higher revs and so you're gonna need a lot of space to get to it's top end. TBH, I think you will struggle to top 160mph with a stock vehicle. That's based on my modified 2.7 Bi-Turbo A6 topping out at 165mph and that had more power, but worse drag.
Hopefully someone with a de-limited TDi can put some real figures here!
I have taken mine to the limiter at 165MPH (speedometer figure, not GPS) and believe it or not, it was on the highway going slightly uphill! I am sure that it would hit 175MPH (speedo) on the long enough straight road. These figures are for my car which is modded.
I am also sure that standard A5 3.0TDI would hit let's say 167MPH (speedometer value) and a veeeery long road and without the limiter of course.
I am still trying to find anyone who is able to remove the speed limiter, no luck yet.