If you're just shooting steel or paper, and doing a lot of it...5.0-5.5gr of Hodgdon HP-38 or Winchester 231 (same powder) in 357 brass will push a 158gr coated RNFP or LSWC to 850 (5.0gr) and 950 (5.5gr) all day long out of a 4" GP100. They barrel and cylinder will also stay a LOT cooler and you'll feel more of a push-type recoil than the SLAP you'll get from Titegroup. That may be slower than you want (being only 38sp velocities), but HP38 is available everywhere I shop, and is more fitting to my needs. Book max for HP38/231 is ridiculous and meant for very soft, swaged bullets. Winchester primers don't even start to flatten at 5.7gr HP38, and use Hi-Tek coated (Acme) hardcast boolits, so no leading!
Edited to add that all this is with regular primers, not magnum.