Your finally getting some dialogue on this one then Nath!
Have emailed my thoughts to you over the last few days, but I think the results from the rolling road day add to an interesting debate.
Jonny on his last S3 was running GIAC, CAI and TBE and regularly saw 345bhp. I think he went for a bespoke map from Regal when he had the HPFP added and I believe he was up to about 360 then (i'm sure it will be in his build thread somewhere!).
The Revo Stage 2+ S3 and VAG-Check Satge 2+ cars ran 324bhp and 330bhp respectively and my GIAC'd car with Panel filter and Milltek donwpipe ran 321bhp.
I think with Revo stage 2+ you're getting a well tried and tested solution.
The GIAC 2+ is less well known and would require a leap of faith.
I'm pretty sure Regal worked with GIAC on refining the maps for euro markets though... so the data has come from UK spec cars