So another data point. I decided to take the same steel bike for Saturday's 100km ride. This time, I pumped the 37mm WAM tires to 45psi (I think i snuck in a few more on the rear and got it closer to 50psi, I admit... baby steps!).
There were a few times on the ride - when we were on really smooth tarmac, mainly - that I felt the tires were a bit draggy. But they also rolled over rough asphalt really well, with no bouncing. I had no issues taking my pull out front at 35-40kph. And looking at my speed/power numbers, they seemed more or less in the same ballpark as my regular bike with 28mm WAM wheels. To be clear, I am not stating the power was identical - but it was more or less where I'd expect to be. I never looked at the numbers and go "whoa, i am going too slow for this power". So that gap wasnt huge.
I am still not convinced that this is the optimized setup if i want to squeeze all the speed i can out of my meager watts. But for brisk-not-murderous rides, it works well enough.
The bright side is this makes me feel better about flipping my R5. Going from 28mm tires on the Venge to the 30mm tires on the R5 wasnt a huge jump - but 32mm WAM tires on 32mm rims, and a second pair of 35-40mm tires on a different wheelset would make the Factor an awesome do-it-all bike.