All else equal, if having less fun improves expected utility, you should have less fun. But all else is not equal, it's not clear to me that the search for more impact often leads to particularly no-fun plans. In other words, some low-hanging fun cuts are to be expected, you shouldn't play WoW for weeks on end, but getting too far into the no-fun territory would be detrimental to your impact, and the best ways of increasing your impact probably retain a lot of fun. Also, happiness set point would probably keep you afloat.