Well if it's an R30 dungeon even having a single R50 there would completely screw your SP gains. That's a big deterrent already.

I'm fine with a cap though but I think as Peregrine said 35 would make a little more sense. It's a dungeon designed for R30 people, not for getting to R30.

As others have stated if it's a cap on stats, abilities, etc... then I'm completely okay with that. I'd rather not be 100% locked out entirely once I go past the cap on any given class.