new rear disks, need more "bite" - any suggestions?

Did you bed the pads and rotors? As in break them in? this is very important.

This right here. I can't stress enough how important it is to bed in the brakes. It makes a massive difference

I follow this method EXACTLY. I suggest giving it a shot before changing anything.

  1. From 60mph, gently apply the brakes a couple of times to bring them up to operating temperature. This prevents you from thermally shocking the rotors and pads in the next steps.
  2. Make eight to ten near-stops from 60mph to about 20 mph. Do it HARD by pressing the brakes firmly, but do not lock the wheels or engage ABS. At the end of each slowdown, immediately accelerate back to 60mph, then apply the brakes again. DO NOT COME TO A COMPLETE STOP! If you stop completely, with your foot on the brake pedal, pad material will be imprinted onto the hot rotors, which could lead to vibration and uneven braking.

  3. The brakes may begin to fade after the 7th or 8th near-stop. This fade will stabilize, but not completely go away until the brakes have fully cooled. A strong smell from the brakes, and even some smoke, is normal.

  4. After the last near-stop, accelerate back up to speed and cruise for a few minutes, using the brakes as little as possible to allow them to cool down. Try not to become trapped in traffic or come to a complete stop while the brakes are still hot.

Here is the full write up:
Bedding in Brakes, Zeckhausen Racing