Here's what I've noticed...A 45 in the "classic" sense (a 7" disk running at 45) will probably not sound as good as the same cut from a 12" 33 LP for a number of reasons (wear, mastering, vinyl quality). BUT, all things being equal, I bet they could SOUND equal. However, if you have two 12" disks, one at 45 and one at 33, and everything else is the same, the 45 disk will BLOW THE 33 AWAY. Unfortunately, 12" singles of the 80s don't always sound better but merely just equal... pressing quality was on its way down.
Again, just one man's observations.