If your goal is really to kill small-medium game to 800 yards with a handy rifle your plan is a good one. I built similar rifle to yours. I wanted a suppressed hunting rifle that was shorter than my old long action magnum 26"barrel+brake. I got 147 eld-m's going 2850 in a short action 22" barrel. Wish it was a little faster, but still accomplishes my goal. Can you squeeze a little more out of a 6.5 prc on a medium or long action? Certainly! Longer barrel will give you velocity too. If you want to explore the outer bounds of what you can do with a 6.5, you could build a 26 nosler launching Berger 156's out of a 30" barrel as well
. Sounds like you already have 7's and 30's for that.
Cheers.