I agree, there isn't much available in a high offset wide wheel for the TT regardless of diameter. I have been looking for a good 18" wheel for track use since October when I got my RS.
I am planning on going with the 1552 Tarmac wheel that is now available in 5x112 18x9.5 et 45 or a custom offset in from et 0 to 50. I am going with a 18x9.5 et 48 maybe 50. The Tarmac isn't a lightweight wheel, but it is a quality, well made and very strong wheel. Ken Block uses these on his gymkhana car. I have talked to the guys at 1552 and they are confident they will clear the TT-RS brakes.
17x8, 17x9, 18x8.5, 18x9.5
Custom offsets available from ET0 to ET50
Custom PCD available
The 18x9.5 et 45 MSRP is $319.52 per wheel. There is a preorder on right now that offers almost $100 off a set.
Ken Block abusing them: Full Link
Here is the back side of the wheels....quality:
1552 had a photoshop contest that I thought was a cool idea: Vortex Thread
A lot of guys in Europe use the ATS GTR wheels in 18x9.5 on there TT-RS. I can't find this model or size in the US though. I believe these are pretty lightweight and would be a great solution if anyone can track them down. ATS Wheel thread on TT Forum