912S low compression 1 cylinder

How long did you run the engine? Run it for about 15 minutes at a higher rpm than idle.  Re-aligning the rings. OR You may have something under a valve. Could be carbon or lead build up from using 100LL all the time. Could be something sucked into the system after the air cleaner. I had a plastic wire tie once under a valve. The owner clipped some wire ties that held the air cleaner in place and a piece fell down into his air box. The cold vs hot check isn't a big deal. Your results should be within 1 psi.

Flew it hard for about 15 minutes then re-checked.  Same result.  The engine has never been run on 100LL.  Would an obstruction be easy to see with a borescope?

Not only you would (likely) be able to spot mechanical obstructions, you would also see colour shades on the valves that may tell a lot. Asymmetrical colour shading, if you find them, are no good.

The piston rings are floating and sometimes they will line themselves up giving you excessive blow by and compression loss. Try flying for 10 hours or so and recheck. I’ve had similar problems that fixed themselves with time.

Made the decision to pull the cylinder.  We're sending it off to Lockwood today.  FYI - I was told that flying it down to Lockwood could cause more damage.  

I'll post our findings later this week.