Yep a free retweak will stop the CEL light, and allow for the dump. You will get a bit more turbo whoosh, but that's about all, all the "noise" per say is handled by the cat back. What you will get is more power and quicker turbo spool. The change in noise I have heard on customers cars after fitting is mild, perhaps because it is 16 gauge alum steel, with a full 2mm wall rather than the very thin stainless walls most dumps have. Still lighter than stock too, given the much higher flow, smaller magnaflow cat.
Yep a dump pipe is about the best thing you can do tuned or not, for a real power increase and to aid turbo spool times, certainly for the price.
Cheers Greg, I've emailed 42 direct as you suggested and all seems good. Just weighing up btw mild steel or 304SS.... Hmmmm.