You should inform insurers of absolutely any and every modification. Premiums may or may not go up depending on the company. When I was with Direct Line, modifying anything, no matter how small would have put my premium up by what I considered an unreasonable amount. I'm with Adrian Flux now and they are much more mod friendly. I had to pay a little extra when I had my spacer, MREV2 and Uprev done, but it was fairly minimal really.
Personally I don't think it's worth the risk of not informing the insurers of mods. If you need to make a claim you don't want to give them any excuse to not pay out!