You're going to get the highest velocity from the slowest powder that will generate maximum allowable pressures and still fit in the case. Lee #2 shows a usable case capacity of .74 CC. That suggests HS-7 or similar to me.
"Maximum allowable pressures" in the sticking point. The weak link is the case in barrels that do not fully support it. It's quite possible that Corbon uses cases that are stronger near the base and running pressures up to 40k psi or so. I do know that some experimenters used cases made from .223 brass to try to make major caliber in IPSC competition. (It's a terrible chore. Don't try it.)
All I can suggest is to start with 8.0 grains of HS-7 under the 115 grain bullet, and work cautiously up, measuring cases for the start of a bulge where the cases is unsupported. I'd use new cases for developmental work and would probably make up a gauge for the cases, just a simple cylinder that a round case would drop through and a bulged case would not. You could also use your barrel as a gauge. Try each fired case twice, rotating 90 degrees.