My experience with 5.5 gr. Unique, 357 brass, 150 gr. RCBS SWC and 158 gr. commercially cast SWCs /standard primers was a chronoed 860 fps. 4" barrel. 5.0 gr. should produce somewhere in the neighborhood of 800 fps.
I haven't tried it with 125 gr. bullets, yet, although I do have some 125gr. Berry's plated HP on my bench. Obviously, the velocities will be higher, and, as has been said, lower charges can cause problems.
I know you can't afford another powder, but when you can, ~ 3.5gr of Bullseye is the standard plinking load with the 357/158gr combination. It would probably also do well behind a 125gr. Just my $.02 worth.
5.0 gr. Unique is max. in .38 Spl. brass, and 5.5 is listed as +P, but works fine.
