I have just put a piper backbox on my furio and the insurance went up £62 for the year... it only cost me £30 extra for the year for alloys
Im with norwich union... all insurers are different.. the £62 i pay allows me to do any exhaust and manifold modifications... so eventually i could fit a full system and not have to pay any more...
Worth calling your insurer though as all of them vary...
Alex