Dont Renaultsport go to a lot of trouble to get the best tyres for these cars? Just using something else that is cheaper and looks pretty seems to be wrong to me.
The tyres are not cheap but then its not a cheap car, so people buying these cars should do their homework before hand.
Think of the warrany issues also. If something goes wrong you might end up putting a set of "proper" tyres on the car to avoid a row with the dealer.
If your thinking of doing some track days why not buy a few sets of proper tyres from a supplier, they may do you a better deal on a bulk purchase.