A lot of people will tell you that as long as the engine is making good power, not making metal and not burning huge amounts of oil, leave it alone and fly it until it needs an overhaul, then do the re-build. This is something well worth thinking about.
If/when you do the rebuild...if you're going to replace the pistons, new rings and a hone job are not options. The new pistons should come with new rings, so use them, and a hone job will help the new rings seat properly, reducing oil consumption, so it's best to do it while you have the pistons out. It would also be a good idea to check the connecting rods or even replace them as well. Cheap insurance...
Not sure, but you may be right about the 8.5:1 pistons being the the most you can go for mogas. I did a quick wiki-check and all the mogas rated variants they listed were 7:1 or 8.5:1.
I think 160 - 170 HP would be a realistic expectation for what you've got listed. To get much more you'd need 9.5:1 pistons and 100LL.
Good luck with the -3,and let us know how it turns out!