Re: .375 H&H 300g bullets best velocity
Ditto to what you posted except burning rate. H4350/ IMR 4350. RL17 and VV 540/550 have about the same burn rate. VV540/550 and RL 17 are both double based as compared to single based powder. Their rates are adjusted to approximate 4350 but their energy density is higher owing to the added nitroglycerin content. Like you I have found they have more energy to develop velocity than 4350. They burn hotter too so it has an effect on barrel life. Probably not enough to concern 375 H&H shooters.
Reloader is denser physically also. Ball powder even more so. In a 375 Ruger case filled to the shoulder/ neck junction (same case for all three):
Superformance (ball)= 95.6 grains
Reloader 19 (double based stick)= 89.6 grains
IMR 4831 (single based stick)= 84.6
You can use this to your advantage by utilizing the extra powder density to shift your burning rate choice to the slower end. By switching to ball powder you can really move slower. Superformance, BigGame, WW760 and AA2700 that I have used in the 375 Ruger so far are no where near 100% loaded density. I am looking at WW780 and slower ball types to fill the case and still reach top pressures.
That has worked for me to develop maximum velocity in the past. Doesn't guarantee accuracy though.