I don't have too much concern about mismatching revolvers and ammo as I have only .357 magnums and do not lend out my ammo due to potential liability problems.
Actually, if .357 magnum pressures/velocities are not necessary, then whatever will noticable drive the comp to keep the muzzle down for a quick follow up is sufficient. Right now I am running 850 fps with 4.8 gr of AA#2 (maybe roll crimp would take that up a bit or reduce amount of powder?) and see much higher levels in tables, so if they can kick in the comp it's good. I'll have an opportunity over the next few days to get a feel for what the loads I do have will do with a comp.
Someone mentioned the progressive shotshell powder. I assume you mean double based powders like 231 ans AA#2. In the larger case the slower powders allow for lower pressure, but what do they do in a .38 spl case?