Spit IIa is now so much more inferior to the 109

[Crumpp]

As +12 boost has a FTH of 10,000 feet this is the power achieved with throttle valve fully open.

Two possibilities.....

If our FTH is at 10,000 feet and our boost is 12lbs then by 12,500 it will be reduced and 10.55lbs is certainly in the ballpark.

OR

12,500 ft was the FTH at 10.55lbs on 87 Octane.

It warrants more investigation.

Actually this is not the emergency power rating

That is exactly what it says and the 1937 RAF Training Manual talks about boost cut out and emergency ratings on RAF aircraft under the general definitions. It revises the old definitions of "Normal rpm" and Maximum Permissible rpm".