My personal best is an indicated 140 in a 210bhp S3.
As for insurance and warranties, it is very dealer specific how they approach these things. My local dealership is anti modification, so I have waited till the end of my warranty. Most modifications come with their own warranty, however, so if you have only a year left it might be worth the risk.
On insurance, I personally would never lie to my insurance company about modifications. If you have a serious accident they will start to dig and no modification is undetectable, what ever people might say to the contrary. However, I am insured with Direct Line, all mods declared (275+ bhp), full comprehensive with Business Class 1 and protected no-claims for about £1150. That's cheaper than I used to pay for my standard S3 with my previous insurer. Nice.
Dan