Looking at the BF 274 data and AH charts:
AH Military Power(+16lbs) speed = BF 274 Combat Power(+15lbs) speed. Good!

AH Emergency Power(+18lbs) climb = BF 274 Combat Power(+15lbs) climb. Bad!

Did the Merlin 61 actually have a +18lbs rating? I have never seen anything to support it. If not, the AH Spitfire F.IX should loose it's current WEP speed.
Ordinance Porkage On Spitfires!Spitfire F.IX:
1. 300l German drop tank
2. 3.5" rockets
3. E wing armament - 2x.50" + 2x20mm
4. Missing 30/90gal slipper tanks
Spitfire VC(3d model is a VB though):
1. Missing C wing armament - 4x20mm
2. Missing 30/90gal slipper tanks
3. Missing 250lbs bombs on wing racks
Since the Seafire IIC was based on the VC, the above probably apply to it also.