1. Changing/adding supercharger gears. Superchargers had some definite limits. One was impeller tip speed. If the tips of the impeller exceed the speed of sound (at least by very much) shock waves form and disrupt the air flow through the supercharger severely limiting it's airflow. Due to the temperature and pressure inside a supercharger the speed of sound is higher inside the supercharger than in the open air outside but tip speeds around 1250ft/sec seem to be a good guide line. If the supercharger is already operating at or near this limit upping the gear ratio will do no good. 2. Dry engine weights vs "powerplant" weights. Dry engine weights not only do not include oil and coolant they do not include starters, generators, exhaust systems, gun synchronizers, vacuum pumps and other odd bits and pieces the engine actually needs to function in the aircraft. Bigger engines need bigger propellers and cowling. As a rough rule of thumb about 2 pounds per horsepower seems to come out well for powerplant weights, a bit lighter for single stage supercharged engines and a bit higher for turbo engines. Navy R-2800s come out at about 3900lbs not including fuel system. 3. Extra drag of some upgrades, Boulton Paul Defiant went from the Merlin III to the Merlin XX and from 1030 hp to around 1200hp at altitude and picked up 9mph in speed. The Merlin XX needed a bigger radiator and the extra drag of the radiator killed most of the extra performance. Maybe with more time and experimentation a lower drag radiator setup could have been worked out. ALL of the extra power of an upgrade or swap is NOT available for extra performance, some of it is used up in extra drag. 4. Other "improvements" needed. The wing or wing group of the Hawk 75 went from 842lbs of the commercial model to about 1000lbs on the early P-40s (The Hawk may have needed a little beefing up any way, reports of wing skin buckling on P-36s) and then to 1120-1130lbs on the P-40E and up. Different gun bays or the wing had to be beefed up to keep the 12 "G" load requirement as the plane went from 5692lbs to 6800-7500lbs (P-40-P40C) to 8300-8500lbs (P-40E and up) ??? or a bit of both??? landing gear weights also changed.