I would expect it is dealer dependent as well.
I dont think there is a black and white answer. I know someone who had a manifold on their car, didnt tell insurance, crashed and they didnt pay out. I know of someone who did something similar and got a payout (through a forum)
If you want to take the risk you may be lucky or you may not. I agree with the more honest you are the better chance you stand.
Give your local dealer a call and ask, they may be ok with it and offer you a guarantee in writing with the map? Cant hurt!