Compressiom
I have an 0 320 with 10-1 pistons, carb engine, balanced, no other mods. I set the timing for break in at 22 degrees on recommendation from one of the bettr known engine builders. After the engine was broken in I reset the timing to 25 degrees. In the super hot AZ weather I climb at 130 knots which keeps the CHT well below 400. High altitude cruise in summer CHT is quite low.
I think the 10-1 increases the power less than 10 hp.
Unless there are runway considerations the best performance is obtained with a prop that will run 2700 r/m or a bit more full throttle at 8,000'.
I don't think I would do the 10-1 again because of the uncertain future of 100LL.
The engine is in a Wittman Tailwind with very tight cowl, much smaller intake and outlet than typical RV.
Whirlwind GA prop, I have to reduce power slightly to stay within the 2700 r/m limit. If I increase the pitch the climb will get pretty dismal in the extreme high temperatures. Temps in the 125 range for several days in mid Sep.