We're using a GHP 10 which is mostly the same thing as the newer GHP 20. We installed the largest pump that Garmin offers. It has no problem controlling our steering to under a couple of feet of cross-track error under all conditions we've put it under. We routinely use it to just steer in many situations. I've come to use it along the ICW in many cases as well. It's the best autopilot I've ever experienced.
Our 53' boat has a displacement of about 80,000 lbs.
I would suggest getting the remote for it - we use that 95% of the time. I'd also get a rudder feedback sensor - we used an N2000 Simrad one. The Garmin autopilot does not require rudder feedback to control the boat so they don't provide one (or didn't when we installed it 2 years ago). I like knowing where my rudder is though. The GHP display will provide rudder feedback position if there is an N2000 sensor on the network.