Why not try a canister and check valve and see what it does? If it fixes your problem you're done, and if not then you can spring for the pump. My logic is, how many times are you under way for extended periods at idle rpm, low vacuum conditions, and then need a panic stop? 99% of the time you're going to be at better vacuum for some period of time, charging your canister, and it may give you multiple brake applications once you're at your lowest vacuum. This is a dynamic system with variables, and your idle vacuum is not that far below what is needed for optimum operation. I'd find out what the canister does for you, and then make the decision on the pump as a last step if the problem is still apparent.