A good way to get sounds generally would be to use a dyno, but these days you can find a hub dyno (rather than roller dyno)
These also have retarders so you can hold specific rpm's (with no slip ratio from tyres, so very accurate), and hold at varying engine loads too!
I'm not sure if they can actually drive the wheels themselves to give overrun noises, but these are likely quite similar to free-revving samples (ie, very small throttle openings)
Of course, the issue will still be other noises present, say the fans in the dyno booth etc, but if you get a sample of the ambient noises in there with the car not running, then that should give you a good base for tidying the sound later (say noise reduction from reference sample in SoundBooth for example)
I will *try* do this with my Z4 at some stage, as they have a dyno like this 5 miles from where I work, and on my way to work, so it's handy. Just £50 for a few dyno runs, so I'd need to probably pay maybe £70 to do 1000rpm full/light load intervals to 6500rpm.
Should give the best results for Racer though... inside and outside specific samples
Dave