I've still got the old SD-5 on the rear and it seems plenty adequate so it's staying. I reckon you don't want too powerful braking at the back, locked wheel etc.
You never know about unfamiliar roads or braking in sketchy weather. I usually try to apply equal pressure front and back, and release if the rear wheel skids. It's easy to survive rear wheel skids. Front wheel skids, not so much. Of course, this works best if front and rear brakes have similar stopping power.