without a remap, performance gains are negligible, so try and declare it as a cosmetic mod. if they aren't able to do that, id either replace the exhaust with a standard one, or find another insurer!
admiral work just fine for me, i'm 27 with 3 yrs NCB, with exhaust and wheels declared its £580.
EDIT - even if you did the whole shebang of air filter, plenum spacer, HFCs, exhaust and an uprev you'd be lucky to get a 10% performance increase!