Sorry, maybe I was unclear on the wording of carrying over NCB. You may have 10 years NCB which eSure accept, but then when you come to move on you find they only give you a letter stating you have 6 years NCB. Not uncommon, may not make a lot of difference, but it also could do and could well be frustrating in the future.
They may charge you £45 now to add the exhaust, but what if you decide to change the air filter? Or the brakes? That could be another £45 each go, if they even accept them. You could also find yourself speaking to a call centre monkey when trying to describe things on your car, and that can be very painful.
Of course, they could be perfectly fine and it might well be a bargain, in which case go for it.