Hi Aaron,
I hunt almost exclusively with the 8x6 SEF coil, and find that it can hit stuff pretty well down to 8" or so. The 11" Pro is handy when there's less trash and/or more time to go slowly, but it can nail stuff down in the 10"-12" range. Depth depends on a number of factors: headphones, threshold, tones, sensitivity, soil conditions, trash, coil, sweep speed, target orientation, settings, and so on. The really deep stuff often doesn't sound much more than a whisper or slight chirp, even in the best conditions/settings, and often not in the classic tones found at more shallow depths. (But man, that quick rising 'squawk' is sweet - gives me chills when I get one that survives on the 90 degree turn).
That said, I have found stuff using the 15x12 SEF that apparently was beyond the reach of the Pro coil (or for any of the other reasons). The problem is that the larger coils make hunting in trashier areas ("signal rich") more challenging (not impossible, just more challenging). If you have a chance to try your Pro coil more this season, maybe it could still find a deep one or two. Less sensitivity actually helps sometimes, as it reduces the 'volume' of signals seen by the coil, without giving up lots of depth. Find the least trashy area of your park and see how much sensitivity the Pro coil can really handle, then work slowly into the more trashy areas. 26-28 manual is good, but 22-24 is fine too. Can also lower the iron mask down to 26 or so to help pick signals out of the iron, but you'll need to rely on the 90 degree turn more to confirm good targets from nails.
Another thing to maybe try is a 'test garden' on site. Bury a Merc or Roosie at 10" in your park and see which coil/settings pick it up best.
HH,
DirtFlipper