My insurance doesnt cover BHP increases stiggy, so having a cat back system isnt classed as a bhp mod, its a stainless steel maintanance free modification and satisfys my insurance companys policy.
If you go for full system, meaning replacing downpipe and a sports cat and the rest of it, thats when you have to declare as a modification as has been said will give you approx 10hp increase.
Although what it does do along with a high flow airfilter is make the car alot more responsive. Improving the torque and not the HP.
meaning the car feels and sounds alot more alert.
Didnt hear the APR RSC last week but would say they are of simular sounding quality from what ive heard.They certainly make the smile factor alot more pleasing!!. I if were you, id save up for the catback if your worried about putting your premiums up, which they dont.