Sorry, I should have specified that the S11 Pro isn't a bad drive by any stretch. As long as you make sure not to fill it past, say, 70% (delete a game or two when approaching that amount, etc) you should be fine. It has just slightly slower speeds than the SP/Corsair drives under non-full conditions - you wouldn't notice it much unless you're looking for it or under specific use cases.
If you paid a good price for your Gammix and you think you can manage the capacity pretty well you might consider keeping it. That being said: I returned mine (HP EX950 - same drive-ish) when I ran across the limitation. Tough decision indeed. You can't go wrong either way, really.