con---big one: overheating. You're not saying size, or your latitude, but with mh lights, a submerged pump, etc, you're going to spike a temp high enough to kill your corals. Plus overheating in an acrylic tank can start it to bow.
I have a 54g with a sump, mh [1] and run at 80 degrees with no lids at all.
I'm a little bewildered by your statement that you "have 2 250w...mh and actinics so splashing isn't a real big deal"---since if splash hits the mh bulbs, they may shatter and harm your tank.
A little clearer picture would help.