Okay - I lied. I decided to get some Conti Race King Protection 29 x 2.2 for my XC bike and I plan on comparing how those ride to the WTB Resolutes.
I keep seeing how XC MTB tires can roll faster than gravel tires, but I keep wondering how much aero drag and added rotational weight play a factor. Guess I'll just have to find out for myself...