Hi Jamie,
     STS spec. etch uniformity (~5 %) is only guaranteed for a
relatively small open area on the wafer. I don't remember what the
specification is though. You might try etching a few die at a time to
improve uniformity, thats what we do for large open area die. Your coil
power does seem high. We usually etch at 600W
     Other factors that might influence selectivity:
  1) Poor thermal contact with the chuck during the etch.
  2) Contamination. If you have opened the chamber recently you might
want to pump it down for several days.

   To improve etch uniformity you might also try increasing your flow
rates and pressure.

Good luck,
    Mike Martin