I love these threads not matter how often they turn up
Warren has answered spot on, if overly worried about your warranty then don't do it but it has been my experience that most dealers aren't bothered as warranty work is very lucrative for them, keep the mods discreet or remove them for your annual service and should be no problem.
From a rmap choice perspective I would suggest stick with main players and don't be lead by price as you generally get what you pay for.
My personal preference after trying, Bluefin, GIAC and Revo is REVO all day long.
Consistent, proven gains with plenty of R&D.
I have an Evolve map on the wifes 320D and it has been faultless from day one with excellent gains so worth considering if conveniently located, also hearing very good things about Shark Performance.
Intake wise, I have tried VF, Evoms and ITG and if pushed I'd have to say i noticed little difference between them, the ITG can look a little makeshift in its build due to the number of components but sound and performance can't be faulted.
Now I have just bought back my previous S3 I will just go with the cheapest I can source out of the above 3 intakes, but do it, the car gains character as per Warren's post.