I have a 24v Terrova with Ipilot on my 17' Tracker Deep V. It's pretty heavy overall, I think max speed is 3.5mph or thereabouts.
I have pulled crankbaits a ton at 2mph with it over years. I've never ran the batteries dead but they will start to get tired after 6-8 hours of running constantly. I have two Everstart 29's.
Trimming up the outboard makes a difference, a lot more than you would think. Although it can get kinda hard to control if the wind is really blowing hard, the lower unit works as a rudder and helps keep things inline.
As for the Ipilot, it's okay. Granted mine is the 1st generation of it, but from what I've heard they didn't really improve until just recently. If you have a steady wind that's consistent from one direction, it'll keep you in a 10ft circle. If the wind speed is very inconsistent or coming from multiple directions it has a hard time, and will overshoot or wrap up the cables, if it does that it will detect the extra drag, unwrap them, and then cancel spot lock. If you're preoccupied with fishing during all this, by the time you notice what's happened you've already blown way off the spot and you get to start over. Aggravating!
Best advice I can give is to approach from straight downwind, hit spot lock a little bit past where you actually want to fish, and don't hit it until the boat has stopped drifting forward. The motor has a predictable "cycle" that you can time your casts with to get where you want to be, and if you timed it right, that will hopefully be where the boat spends most of it's time as it's drifting back and forth over the spot.
It's pretty rare that I use Spotlock alone, usually I will switch back and forth between it and just driving it manually with the remote.
I have never tried to record a route, but I do use Autopilot occasionally, which is pretty handy for trolling. Wherever you point the motor is where it will go, it'll automatically correct for cross winds or current and keep you on a straight course.