[Kachok]

True jmr, but I still use slower powders for longer tubes, not for speed reasons but because muzzle blast gets pretty at times, I tried a heavily compressed load of RL22 in my 22" 30-06 and the blast felt like it would take the hair off your arms. My IMR4350 loads are not as fast but they don't ring your bell like that one does.
The 375 Ruger like most centerfire cartridges will gain speed well past the 20" mark no doubt about it, (even the mild 308 keeps gaining speed until at leased the 30" mark) but that does not mean that it is handicapped by a 20" much either, for a dangerous game rifle being "handy" can be a life of death thing, I'll trade a few fps for that hunting in the brush.