OK had a go with air brakes today.
1mm air brake felt great but even moderately long cruising at 1/4 throttle was pushing CHT's of 350 and I decided to go to the 0.6mm.
Installed 0.6mm and went for a ride where I knew I could hold 1/4 throttle for a few miles in a row. CHT's up to near 400 when I pulled off and let it cool down. it was a long slow steady climb in CHT, but there is clearly not enough fuel at 1/4 throttle with the 0.6mm.
I'm going back to no air brake and the 98 main instead of the 100. This is the setup I was riding before trying all the air brake stuff. To be honest, I probably need to take this setup over to the same stretch of road and have a go to see if the 2.6 needle bushing is too lean. I'm pretty confident it's OK, as I've never seen temps climbing too high at these throttles (I've seen 320's) but it is worth checking off.
I'm glad I found that stretch of road as a 1/4 throttle test. I can assure you it will come into play as a testing ground on my P200 jetting.