I guess I'm confused then. If you're level 50. And you've beaten all the content in the game. Multiple times. Why are you still playing? Is there some leader-board I don't know (or obviously care) about? How exactly would such a thing be remotely relevant with client side, editable saves; loot drop hacks, etc?
If you're still having fun, then cool. But clearly you're not, or you wouldn't be raging about upgrades. You say it yourself, it's not an MMO. Put it down till the next DLC hits, grab one of the other 80 games you got from the holiday sales, and play them for a while.
Since subscription MMO's have a monthly cost, I can see (but don't necessarily agree anymore) making one of them your main, your only game. Farming EVERYTHING, getting all the achievements, levels, gear, whatever. Spending time BSing with friends, or not.
But BL2 *is* not that beast, it does have an end, no reasonable person will vilify you for putting it down, be it "for a while" or forever.
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As for the creature slaughterdome DLC, it's just the critter version of the hyperion dome and bandit domes. If you enjoy that game play, give it a go. I'm personally waiting for it to go on sale because dome's really don't give you that many hours of play.