Lease companies might tolerate a bit of sign writing, but would they let you convert it into a race car on the understanding that it had all its road car bits fitted when you handed it back? Not a chance, they want that car back in tip top stock condition so they can sell it on, approved used, with a warranty. They certainly won't be to keen on you risking the future value of the asset, and may even consider it such an abuse of the vehicle that they insist you buy it from them outright. Your contract will state what they consider to be fair use, and I doubt voiding it's warranty is included as acceptable.
VWFS has also been shown to lack understanding when it comes to mods on PCP cars. This is perhaps the most infamous example, but there are others;
http://www.seatcupra.net/forums/showthread.php?t=193416
In short, if you don't own it, don't modify it.
There is a manufacturer approved, warranty friendly and very effective way to get more power out of an S3 though, so all is not lost. You simply need to buy an RS3.