24.0 gr of H110 under a 270gr Keith does 1,100 fps from my 5.5" Bisley. You can use the same load under a 255gr LSWC with similar results. This has been very accurate in my guns, some shooters report 1" or better groups at 25 yards with this load.
7.5gr of HP38 under the same bullet averages 850 fps in my Bisley and is one of the most accurate loads I've found for that gun.
Neither of these is a real monster load, I generally go to higher bullet weights when I want more power instead of pushing the light 255-270gr bullets to higher velocities.