Back from the dead, eh?
I think for the average person, this could be a great bike. For a lot of rides (including our recent MI coast to coast), fat tires will be the ticket on roads/terrain where 40mm isn’t quite adequate (sand, rocks, root, loose gravel).
The 15-18mph range covers a lot of gravel rides. At those speeds, its rolling resistance (and weight) that makes the difference. A nice big tire is going to give less rolling resistance on a rougher course.
I did an experiment with a $300 30# mountain bike on excellent tires vs a $2000 20# bike, and found the speeds were pretty comparable on a ride that was fine for 40mm tires. Degrade my course at all and the bigger tires would win. The one drawback of the heavy bike was that my endurance on a 3 hour ride really took a hit – the last 35 minutes of rolling hills was hard on the heavy bike.
Wheel/tire weight makes a big difference for me on short punch rides – less so on long steady effort rides. That said the “gravelcyclist” calls tires like the 622-50 Furious Fred an “unfair advantage” (if you can fit them).
Bigger tires are always easier. The question is – are the routes you are doing pushing 40mm tires past there limit? Are there sections where you have to really slow down or be cautious on 40mm tires?