1300mm sounds like it should get a pretty decent shot. In town, he would have to stack shots. I don't remember if the T-ring only fits one size extender, but I think that is so.
I also have a 10" water heater on a GEM mount, but
haven't used it in so long, I'm not sure it would do good any more. Not
sure I could use the 2" system on it, either. It takes even longer to align