I don't dispute your findings at all.not too sure about that.
best bids i've ahd for mine through independant trade is 28500. they generally price 2k behind audi retail so i'm figuring this is why.
Maybe it's the difference between a straight sale to an independent and a px to Audi?
If Audi dealer has £8k margin in a new RS5 perhaps he can afford to blow a bit overallowing on your px?
Someone who had just bought a V8 Vantage told me the dealers had £20k margin on them so were overallowing massively on all the cars they had offered in px (he had a 911) because they could.
As a general rule it is worth making a few calls as even among Audi Centres their offer price can vary massively.