the manual states 5 min max, this really means you should get 'at least' 5 minutes before engine damage starts to occur.

Provided the safe coolant and oil temperature limits are observed / can be maintained. Given that +12 lbs develops 30% more power than what the cooling system was sized for, there may have been issues with that.

Are there any cooling trials for the Spitfire I/II?

I suspect I am doing something wrong tbh, the behaviour of the power, RPM and mixture is different than before so it's like learning as a beginner.

Sort it out before Mission 2 begins, man!

I don't know of any myself but then I am not a great one for collecting data, annecdotally the Spitfires had no issues with cooling once airborne and would maintain stable temps at full throttle with rads closed, so I see no reason temps would change the 5 min limit, I think the failure times should be randomised somewhat to make it much more like real life, but with 'hard coded' limits you just get an engine that goes 'bang' exactly on time, let's face it, there are many many stories of aircraft returning home with massive amounts of battle damage to the engine and they kept running and they certainly werent designed to run under those conditions.

5 or 3 minutes? I remember manuals saying 3 minutes. Now is 5?

Are these new engine variants so powerful that its possible ATAG may limit them on their server in the name of "balance?"

Ah! Ok! I checked:

5 minutes to II A

3 minutes for the I

They should use less IIA not for balance but for historical accuracy. IIA only comes at the final o the battle and with reduce numbers.

Most spits shoud be the IA.

Plenty of cooling data in the respective Spit IIA AVIA files in the archives. The Devs have these.