Nice Question as a 1st post, first of all, Welcome to the Forum!
We're a sensible bunch on here, so YES, you need to tell your insurance of any BHP tweaks to your car, just to make sure you ARE insured when driving it!
Does it whack the price up a lot? Depends on you and where you live, I'm paying £450 fully comp with all my mods declared and 6 points on my licence, would have been around £300 if it was standard I think.