Viper - saw your post and went off to test it. You're right about it changing the propellor pitch - I see the RPM drop from 2900 on takeoff to around 2400 at about 1200 feet and stay there, and after that the Spitfire accelerates/climbs like an absolute pig. I'm getting climb rates of around 110 mph, not 170, though that is low level after takeoff.
It's the difference between the two that's most startling. I tested the same in a Hurricane at 2000 ft and was getting a climb speed of around 170, rpm again in the 2900 region. Is this really just a quirk of the aircraft, or am I just flying it like a pillock? I had no trouble at all in the Hurricane, I assume because it doesn't have the variable prop?
Thanks for taking the time to check for me anyway.
I realise I've repeated the same circular argument that the op began with, although I guess I now have a clearer understanding of the way the engines operate.