The Mk IIs were a bit heavier but the weight hides a few things, like the IFF gear which required an Antenna on each side of the planeMK II was a more powerful but slightly different plane, on most metrics of speed at altitude it appeared to be worse than the Mk I, but this was because it was optimised to perform better overall, which it did.
Gear weighed about 40lbs and the extra drag slowed the plane by about 2mph (?)
Armored windscreen, rear view mirror?
The MK II did have a bit more service ceiling than the MK I despite the extra weight.
I am making a guess that the supercharger may have been mispatched between the intake and the gear ratio? The intake of the supercharger may not have allowed the flow desired using the higher rpm of the impeller. They might have gotten the desired pressure but if the supercharger was operating at lower efficiency and heating the intake charge more they might not have gotten the corresponding increase in power?