There are no downsides at all to such patches - if something is truly immutable we want it const, and we want to propagate that information through all call sites. It improves security and it improves code readability as well.
It would be nice to push such grsecurity patches upstream. If you'd like to help out splitting things up and pushing them upstream, it would be a very useful effort. (Maybe the grsecurity author(s) will help you out with that as well.)
I'd suggest for you to start with the simplest, smallest and least controversial changes - the best way to learn the ropes.