There are no "Perfect Miters"; the notion begs the question: perfect in relation to what? The back of the molding or the front; if the back, for what portion of its length? Frequently, the lip of a molding isn't perfectly parallel to the molding's back. What you want is the best miter you can get working with imperfect material and somewhat imperfect equipment. I honestly don't think a chop saw that you swing from right to left miters will give good results. Get two saws and lock each at as close to a perfect 45 degree as possible right and left and then never move them.
If you can't get good cuts from your present set- up, perhaps the easiest solution would be a good miter sander; nothing is going to produce a better miter than a sander. But as I mentioned, there are no perfect cuts; every corner has to be tweaked to an extent. If before you cut the miter, you could run the back of the molding over a jointer and then could plane the lip parallel to the back assuring that the the back and the lip are straight and parallel to each other, you'd have a good chance of cutting a good miter. How many feet of molding are straight? Sure you can hold the molding flat against a fence as you cut it and then the miter will be close to exactly 45 degrees to the back, but only until you let go.
We have the best saws and joiners money can buy, and perfection conststantly elludes our frame maker (12 years experience doing nothing else 8 hours a day).
You need a strategy for imperfect miters. Ours is the use of bar clamps (we use Bessy K Body clamps-we have about 40 of 'em in varying sizes, 2 are a little under 96"). Once you have v nails in 4 corners of a frame, you can do a lot to tweak its corners by applying 4 bar clamps. Bar clamps can
apply orders of magnitude more pressure than the nearly useless web clamps and allow, with the addition of matboard spacers, a great deal of control. I've yet to see a corner (of a completed frame) that the clamps can't save no matter how far south it has gone. In a run of 30 different frames, it's not unusual for us to have to resort to the clamps on 4 of them. The clamps and glue make perfect corners but not from perfect miters.