I have taken a few subarus off the clock on the autobahn.
The big factor isnt hp it is the gearing.
What will 5th or 6th allow you to do?

 

At Elvington I was clocked in my 95 WRX at 176 mph and guess it was well under 400 bhp at the time and in the 97 STi wagon at 168 mph a long way before the timing trap and that was around 360 bhp at that time. Just keeping it close to the red line which I do not like doing on these cars.

No. Timing trap figures. top speed runs.

Not standard gearboxes, diffs or wheels.

Generally on stadard wheeks and tyres these cars over read by 5-10% subject to model year.
Unless speed is constant I (and road is straight and level) I am not sure as to accuracy of some GPS.
I have an SPA microprocessor in each car and it logs speed to absolute accuracy or as accurate as the input data. That is how you can do a genuine 50 or 70 mph with a traffic car on your tail, pass everything and not get pulled or as did happen to me get pulled for a routine check, "what speed were you doing"
Bang on 70 mph
Show them the eqipment, comment that you sre also data logging all sorts of info, including speed, Don't show them what you were doing half an hour before.............
"What is that in your tail pipe"
Show them under the bonnet, explain a few things and ask them what their deisle BMWs are like and part cordially.

 

122gps, @ 6000rpm

 

I accidently hit the limiter in 5th in my v1 wrx sti. Clock was reading about 157. Running region of 280bhp give or take a couple of horse