@AerobaticAce As promised I’ve investigated your concern with the transfer pumps and done three identical test flights, after implementing a float valve simulation to prevent the main tanks from ever being filled to max capacity during transfer.
All tests were in the G-111 Long Range, at 2000 feet, 72% throttle, 2300 rpm, 30" map, over 2 hours and 6 mins (no time compression) at 135 gallons per hour average burn, starting with 100% full tanks and flying an identical profile with no weather and no wind. Transfer pumps were set to drop tank → main tank immediately after flight start, and simvar watcher was used to track the values.
Test 1 - Transfer pumps ON, no float valve simulation: 284.61 gallons used
Test 2 - Transfer pumps ON, float valve simulation turned on: 286.11 gallons used
Test 3 - Transfer pumps OFF: 284.39 gallons used
All three tests ended up with nearly identical fuel burns, certainly within margin of error given that a takeoff and climb out were part of the profile.
I won’t be adding the float valve simulation as it doesn’t seem to be needed and it could potentially cause unforeseen issues later on. That whole “if it ain’t broke…” thing. 
Thanks for bringing this up though, was absolutely worth testing to see if there was an issue here.
Cheers!