I realise that this thread is about gravel racing, but perhaps people following this thread may be interested in some analysis I did of my cyclocross results this past season, for which I occasionally used my drop bar MTB. There are no tyre width of bike rules for my local CX races, so anything goes.
For the six races I did, I looked at my finishing place in the field, as a percentile, and also my time behind the winner as a %. For two of the six races I chose to use my drop bar MTB, with 2.35” Thunder Burts. The other 4 races were too muddy, so I used my cyclocross bike with 33mm mud tyres (Challenge Limus or Baby Limus, both HTLR casing). Most of the field used CX bikes and 33mm tyres for all the races.
Ignoring for a moment my rather mediocre results in these races, it’s interesting to see my best results were on the bike with MTB tyres. There are obviously multiple factors affecting race results, so it’s far from conclusive, but I think it’s still a strong indication that the MTB tyres are faster for those drier races.