I have a couple things bookmarked. There’s also data for 700cx23 vs 700cx32 so we have a good idea of scale of difference just not actual numbers [that I could find].
Dylan Johnson’s wind tunnel video:
And this from Swissside
The measurement data was very consistent with both the Schwalbe and Continental tyres tested. The differences per 10mm increased tyre width, were around 3.6W at 30km/h, increasing to almost 12W at the higher speed of 45km/h.
Swiss Side Aero Tip - Gravel Tires Width
A quick search hasn’t shown much else (that hasn’t been posted earlier like Torsten Frank’s excellent analysis) for larger MTB tires in gravel frames/drop bar MTBs.
I do wonder if there is anything interesting going on with frame/fork clearances wrt tire width. Say a gravel bike frame and fork that fits 700cx57 and optimized for aero at that tire size, is going to have a much different aerodynamic profile fitted with tires 700cx42 - which would probably be frame/fork/tire(s) specific. This is tangential but I think helps understanding the overall model.