I wouldn't go any higher than a 3/4 HP. In fact, I am running a 1/2 HP (full rate) pump with solar and a 25' roof without any problem. The panels prime fine in less than about a minuite.

Also, I am surprised you need to restrict the returns at all. You shouldn't need to do that with only a 25' rise. Again, I am able to prime my panels with about 15 PSI. Have you tried it without the restriction?