I've had 168mph out of mine with the GPS. The speedo was off the needle too at over 180. 
Depends how inaccurate the speedo is at a guess. I reckon that clip was a solid 170mph on a GPS, which would put the needle off the end of the speedo 
I had about the same
downhill on the German autobahn. I remenber, even if I realy had to focus on my driving, that I saw 275km/h (about 172mph) at the GPS and the car was still going a little bit faster. Then I saw 7200rpm on the tachometer in 6th gear that is about 290km/h (180mph) at the speedometer. That's bloody fast even more on a bumpy two lane higway, with curves!!!

My car is about 300Hp
That said, I made some simulation based on real A3 speed recorded with several type of engine ans it seems that maximum real speed achievable with an A3 on flat road is:
300Hp => about 260km/h 160Mph
350Hp => about 270km/ 170Mph
400HP => about 280km/h 175Mph
To reach 300 real km/h 190Mph you will need about 600Hp!!!