Floorpan wise the tunnel is exactly the same. The floor pan is the same up to the front of the boot floor so where the rear seats are.
It has been done, but cost about 10k to do it to factory standard using a new 4Mo rear floor (seen it done on a Beetle) but you have to factor in tank, pumps, etc. You could get a front ended S3 and unpick the rear chassis and attatch it to yours, if done by the correct people it would be just as safe as the original if not more. Done a very similar operation building WRC cars that arent meant to be 4WD, but it will cost far more than the car is worth, might be easier getting a Leon 4 bodyshell and converting LHD to RHD.