Well, considering Superman is now Pre-Flashpoint [i.e. OWAW Superman] and New 52 Superman in one being, I'd bet on him going all-out from the get-go, knowing what happened the first time he didn't, and with better results considering he is more powerful + experienced.
Make it pre-merger, and we'd have a topic.
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“Perhaps this is the ultimate freedom. The freedom to leave.”
Much better fight, of course... Though it seems like universal/multiversal threats are relatively commonplace in the DCnU, so I have no doubt the heroes would ultimately find a way to overcome Imperiex.