Traveling between city and airport isn't that much of a hassle, immigration is usually a snap (esp. at mildly unusual times like early in the morning when you can jump the queues as J passengers) and taxis to the city take less than half an hour in good traffic (again, 5-6 AM is pretty good). The primary negative would be having to pay the 500 baht (~US$10) airport tax on departure.

That said, 2.5 hours is a decent safety buffer, it gives you some room for error and is not a terribly long time to spend in the airport, esp. since as J passengers you can use the lounges (I presume? not sure how UA's upgrades work). So I'd think you can go either way.