Great stuff, you must be excited about picking up your RS3.
All the usual mods, the sort of stuff you did to your S3 are available for the RS3, but I'm guessing you might know that already.
The interesting thing with the RS3 is that a stage 1 remap for between £600-£900 from the usual companies will get you anywhere from 405-414 bhp with no other mods. Most standard RS3s seem to Dyno closer to 360 bhp than the Audi quoted 335. (Oh and If you want Stage I with a warranty (of sorts) ABT Sportsline work with several Audi dealers but it costs nearly £5k.)
Stage II by contrast costs about £3k more to add a sports exhaust and intercooler. For which you are likely to see 425-430 bhp. That' a lot of extra cash for 10-15 bhp!
A few things people do seem to go for:
Secondary cat bypass pipes - removing the 2nd set of cats, most insurers seem happy. You get more induction/turbo hiss and a deeper growl. Cost effective for extra sound.
Tyre upgrade - I fully recommend this, once the standard Contis are shot. I went for Michelin Pilot Super Sports as they have an edge/shoulder compound that resists wear well (you'll find the Conti tyre shoulder wears off quickly). The MPSS have lasted road and track time well. Others (inc evo magazine on their RS4) report similar improvement on Goodyear Eagle F1s. More steering feel, less understeer, more adjustability, more grip, better turn in, much improved traction, flawless launches - lots to like from just from a tyre change.
The only mod I have is front AP discs and Pagid RS4-2 pads, brilliant but pricey at £1500 and only really needed if your heading out on track (the standard discs are like chocolate on track).
Other than that the suspension options are much the same as the S3.
That's about all I know to tell you about but I'm sure others can add more.