Unfortunately, when I had mine the slowest powder we had access to was 7828 and I took that last barrel off well before powders like Retumbo, H50BMG or N570 were available.
I would guess using powders this slow would gain between 50-150fps, but this is a WAS at this point. Mine had a 8" twist even back then, but the factory barrel was 9 1/4" twist which lasted 600 rounds.
I got high velocities with 175g bullets, 100fps faster, with RL25 when it came out. No other powder then could match it in several magnums.
Someone must be shooting one with 195's and slow powders here.
Cheers.