That is a very hypothetical question. You're asking if the insurance premium you don't have, with an insurance company you haven't approached, for a modification you haven't done, on a car you don't have, will be increased?
Company 'A' might say that you have improved the braking performance of your car so are much less likely to have an accident and your quote would be less
Company 'B' might say that because you have upgraded your brakes, you have increased the speeds at which you're likely to drive, are then more likely to have an accident, so they will increase your premium
Oh, by the way, company 'A' doesn't exist , or if it does I want their number!
On a serious note, some specialist insurers who deal mainly with modded cars won't necessarily put up your quote just for upgraded brakes, but the big companies probably will.
Not 100% sure on the DVLA, but I would say you didn't have to inform them, it's only for major things like changing the colour etc. (or anything else that is stated on the registration document).
How long is a piece of string... on a modded FIRE 8v at best it was a neutral thing, for 16v units it was neutral but anything involving N2O or a turbo it reduced the premium some what. For an N2Oed and turboed engine it was mandatory & oddly enough if I fitted a roll cage it was mandatory to upgrade the front brakes no matter what the state of the engine was.