Well, argument on summertime conditions is pretty much pointless.
The FTH depends on pressure altitude so at summertime conditions the FTH for given manifold pressure actually increases in the density altitude scale. In other words, the full 12lbs boost in summertime conditions is available at higher density altitude than at standard conditions.
Physical explanation is fairly simple: The supercharger increases the density in manifolds by keeping the pressure constant up to the FTH. In summertime the warmer air is less dense than in standard conditions so given manifold pressure gives less power than at standard conditions. However, the ability of the supercharger to keep the pressure depends on pressure difference between manifolds and outside atmoshere (plus dynamic pressure) and in summertime the pressure at given density altitude is higher than in standard conditions, hence the FTH increases in the density altitude scale while staying constant at the pressure altitude scale.
All this can be found from the USAF Flight Test Engineering Handbook, available from Scribd:
http://www.scribd.com/doc/19204672/U...ering-Handbook
The problem here is that some members have stated something else and they can't never admit that they were wrong...