Get new pads first, you can pick up 4 sets (8 total) of the tri-colour/compounds for less than £10 on ebay.
If they don't have you stopping with V's very quickly, especially on the rear then something else is up. That rear should lock up with little effort.
Worn, used, glazed pads will feel like the back end is just gradually slowing you down.
I only used Brake Booster (odessy U4) on my MAGs forks but then the design there is also to stiffen them (forks) up as well as improve braking (where it works a lot harder) due to rotation of the legs on braking.
On the rear all it really does is let you improve modulation a touch from the feel.