The only thing I typically shoot pre-mounted is trap. Once in a great while, I may shoot a particular sporting clays presentation that way, but it's very rare.
Last week I shot a course which had a devious little target. It was a long incomer, off to the right, which passed between several sections of trees. The first "window" it reached was a long ways out. The second one would have been preferable, but it was very narrow. The final opening was almost directly across from the shooting box, maybe 20 yard away... and by then the target was dropping fast. My first shot was the long one, and I got a hit, but it was really chippy. The next bird I let get to the last window. However, I'd been tracking it for so long, and it was travelling so slowly that when I did shoot, I wasn't swinging the gun anymore. Miss. So, I decided to wait. I called for the bird, and then just kept the gun pointed at the sky until it cleared the last tree. I then swung the gun up and when it hit my shoulder, I pulled the trigger. Dust. Same with the second in the pair. Same with the next pair.
The guy I was shooting with is new to clay busting, and his background is mostly deer hunting. I watched him miss the first pair, after following the birds like I had on my miss. I told him to wait until the bird cleared the last tree, THEN start his swing. He couldn't do it. A little before the clay got to the tree, he raised his gun and carefully followed it. He stopped his swing, shot, and missed behind and above. This time, I explained it again and told him to ignore the bird until he could see it dropping past the tree. He did, and it shattered. He then went on to do the same thing with the next pair.
It seems that most of the misses I see, especially with newer shooters, comes from two areas. The first is not being on the correct line, and the second is stopping their swing. Some coaching and practice will get rid of the first problem, and a low gun will take care of a lot of the second.
BTW, a great hunting season prep is to shoot sporting clays in the following manner:
1) Low gun, safety ON.
2) The trapper decides when to throw the clays, not the shooter.
3) If the station allows for it, the trapper decides whether to throw a true pair or a report pair.