It's because the Spit V is the new diamond in the rough (not to mention the boost is at maximum at all altitudes and times (16 pounds/100 oct). Thing accelerates like a cheetah on a fat gazell.
I do believe the boost on the Spit V is a single stage supercharger. There were several variants of the Merlin 45 series engine, a low altitude and a high altitude variant where the boost was calibrated for either 1km or 4km in altitude.
Seeing how both the Seafire and the Spit V we have boosts to 16, it means Pyro/Superfly has modelled the Merlin 45 which allows 16 lbs at all levels. The 4th Fighter Group has some good information with actual test results here:
http://www.fourthfightergroup.com/eagles/aa878.html (Merlin 45)
There is of course differences in boost available during climb and at level flight, I'm not sure if this is modelled. The boost on the SpitV may need to be tweaked a little I think for two aspects - boost at climb, and boost at level flight (all related to speed) or at least explained which version of the Spitfire V engine we have... Pyro/HiTech/Superfly? Also, the limit for boost of the Merline 45 A.A 878 engine is only 3 minutes before maximum engine temperatures are reached.
- Lumsden, Alec, British Piston Aero-Engines and their Aircraft, Airlife, 1994.
- White, Graham, Allied Piston Engines of World War II, Airlife, 1995.
- Rubbra, A.A., Rolls-Royce Piston Aero Engines – A Designer Remembers, Rolls-Royce Heritage Trust, 1990.