The L frame is the same size as the GP100. The 8 shot models are an N frame.A 3" SP101 or a 4"GP100 makes a great all around gun. I don't really see the point in going with an L frame smith for .357. They do make 8-shot models which are cool, but it is a huge gun for such a small cartridge.
I shoot my SP101 almost exclusively with near-max H110 reloads. It hasn't showed any sign of loosening up. I also don't think it hurts to shoot. It will give me blisters after awhile, but no real pain. Most recently I settled on some Badger Boot Grips after trying nearly everything out there. I like those a lot.