Sand,
I looked into this fairly extensively in the past. I have a 2000 Hunter 340 and have had the same problem you note, namely that the wheelpilot works only in mild wind conditions and then can't turn the wheel over in anything much above 12-15kts. I've also had the wheelpilot shred its gears a few times trying to fight confused seas, but that doesn't seem to be a big problem for most others.
Anyway I've looked into how to install a below decks pilot on a 340 and I believe the only thing that would work is a Raymarine linear pilot, but not without serious surgery to the floor of my cockpit where the steering quadrant is located. Basically it would require cutting into the floor, installing a watertight "boot" (like what is around a manual stick shift in a car) and allowing the part of the linear drive to stick into the aft stateroom. Long story short, I haven't done this simply because it seems like way too much work and way too much modification for an improvement on the wheelpilot. The 340 unfortunately is too big to effectively use a wheelpilot but not large enough to allow a below decks unit to added effectively.
Boats are all about compromises right?
Take care,
-Levin