I am unfamiliar with the gun you have and it's strength. Ask the smith who worked it over what pressure levels it is capable of.
Since I assume you want to stick with loads around 900 fps, I would consider 300 gr as the max, and perhaps 280 grains better. Understand that heavy bullets at the same speed as lighter give a harder push than lighter, but it isn't a dead slow push, or easier on your hand. At a given velocity, heavy means a heavier snap of recoil, and the lighter the gun the worse it gets. 300 gr kicks approximately twice as hard as 150 gr with both fired at the same speed. I personally prefer to take the punishment of recoil to carry a light revolver.
I'd throw out the Unique and switch to Hodgen Universal for similar speeds. There are other new powders which I hear are equal to Universal but I've grown very fond of this stuff, and did it quick!