I have hit level 50 and the game is dropping level 50 guns. Are these guns as good as they get?

Yes, you can spend your golden keys and cash now, a level 50 gun is a level 50 gun. Playthrough 2.5 is only important insofar as it makes getting level 50 guns much easier.

Playthrough 2.5 is important for two things:

• Scaling Quests (and their rewards)
• Scaling enemies

See How do subsequent playthroughs work? for info on quests (you're clear to take quests now, as long as they're level 50), but the main thing about playthrough 2.5 is that is scales enemies to level 50. Better stuff doesn't drop in playthrough 2.5 per se, it's just going to be (up to) level 50, so it's the best stuff. Technically you can go back in playthrough 2 and find non-level 50 stuff which will still drop lower 50 guns.