From my experience with both a MAC 12T and 10T both are stout motors at lower speeds. 550 lbs on climbs could strain the internal reduction gears & clutch on climbs.
Starting with efficiency, the frequency of pole switching events has the largest effect at lower speeds. Virtual rotor speed. You can get an idea of that about halfway down in this article:
http://www.triketech.com/Drivetrain/Pow ... AC-V2.html
The Grin simulator is an exceptional tool, when used for comparisons but in the real world one finds their usage often deviates from what they were inputting.
From my testing at 52V @ 20 amps the 12T delivers more torque than the 10T up to about 12-14 MPH and the 12T thermal rise isn't as fast as the 10T. Above 16-17 MPH the 12T torque begins to taper off a bit below the 10T. Above 18 MPH the 12T thermal rise is slightly higher than the 10T. That's from testing on hills of 7-10% with a 265 lb trike.
I can't tell you if that agrees with the simulator, so much as that's real world results.
Internal gearing has its limits. The 12T at 40 amps would spin the tire off the line. It also made a few dreadful sounding noises that sounds like gears stripping, but it was actually rotor slip which can occur under high loads when the stator leaves reach saturation. In the short time I ran at 40 amps the audible noise level from the internal gears increased a bit and even with reduced current the 12T is now louder then the nearly silent 10T. That testing was done with the same Infineon 12 FET trap controller, I now run the Phaserunner with the 10T. You will be better off with the Infineon 12 FET trap controller.
Your looking for hill-boosts. Here are a few numbers to start with:
5MPH on a 5% grade will require 274 watts of output just to lift. At that speed the efficiency will be much higher for a geared vs direct drive nearly double. Less heat to shed.
Required output power is pretty much a linear increase to speed or grade. So at 10 MPH on 5% or 5MPH at 10% just double the watts required. But here is where it gets challenging; how many watts in to get the required output depends on efficiency which also depends on the speed.
Limits. You have a 36V battery that I would assume (LiFePO4) capable of 30A continuous (check, including BMS). That allows a budget of about 1080 watts. If you pedal at about 30-40 watts (overcoming rolling/areo drag) at 7 MPH climbing a 10% grade the efficiency of the MAC12T will be in the 68-71% range; that provides about 750 watts to the ground, and 330 watts to heat. Most typical DD motors will be down in the 50% range and below in the real world.
Trouble is at that speed it won't shed heat fast enough for sustained rides, and you'll be building heat at a rate of about 300 watts. A simple rule to convert that to thermal rise is every watt-hour of heating will raise 1 Lb about 3° F. So that begs the question of how tall is the hill?
Lets work with 100 foot climb increments. 7 MPH @10% will take 97 seconds. 97/3600 = .027 hours, multiply by 300 watts = 8.1 watt-hours = 24 BTU. The stator assembly weighs about 5 Lbs, so 24/5 = about 5° F.
These numbers are part of a spreadsheet that I've been developing for a few years now from real world testing. It does deviate a bit from the simulator which again is an excellent comparison tool, but somewhat limits the user in accurately inputting/defining real world application.
Bottom line is I would recommend the MAC 12T at a 30A max limit with the internal temp sensor connected. It wouldn't hurt to have a spare set of planet gears and clutch on hand, if it does fail you can still pedal (or push) home but you won't be stranded for long. Gear life is a bit tricky to project as the AGMA gear wear rate formula depends a bit on the tribilogy of the nylon gears and that's not a linear value. My best guess is you'll get at least 1000 miles under worst case, but probably closer to 2000 miles at the 1080 watt limit.