Any ideas on how much (if any) an aftermarket backbox increases power?

I've put a Scorpion SS BB on a MY00 classic, to improve the sound of the car. I've declared the mod to the insurance company, who want to know the power increase. Now I'd always thought that BBs didn't really affect power output without the centre section and downpipe also being changed. Is this correct?

What should I be saying to the insurance company?

 

Prolly 5-8hp at most