Actually, a lionfish will often eat to the point of regurgitation, which is not good. Additionally, since lions have slow metabolisms, too large a meal, or too many meals in close succession may result in the food items beginning to decay in the fish's GI tract before it can be digested, which will kill the fish. Finally, to much food or the wrong foods can lead to hepatic lipidosis, which will also shorten the lifespan of the fish.
Best bet is to feed sub-adult and adult lions 2 or 3 times a week, and only feed till you see a nice little bulge (not a huge one) in its belly. Juvie lions can be fed a little more often so they can gain a bit of size, since much of their caloric intake is used for growing. We feed our juvies their "main meals" 3x a week and they get a few small feedings on some of the "fast" days to give them a bit of extra nutrition. These "small feedings" are simply a bit of thawed mysis/BS+ dropped into the water column while I'm feeding our "daily eaters".