Raster,
The empty weight of a Mosquito is indeed higher than that of a P-38. It was not just made of balsa.
I bet that just the second pilot with all his accessories would be a few good hundred pounds. Though de Havilland saved a few lbs on putting less armor around the navigator than the pilot. I remember McIntosh complaining about this (Terror in the starboard seat).
The overall weight of a wood contraction was similar to that of metal for similar strength. That was the claim by de Havilland. However, the wood skin offered a few advantages to internal structure: since the skin is much thicker and less flexible than aluminum, it requires less support from the internal structure leaving more usable internal volume.
In Mosquito by Sharp & Bowyer the development section gives the impression that while top speed was of interest, de-Havilland were much more concerned about maximum cruise speed - that is the fastest speed sustainable over a long distance. "Speed" for then was how long it takes to make a round trip to Berlin, not where the IAS needle pegs. As a "fighter" it is considered first and foremost a night-fighter. There, interceptions tend to be long and ninja style - the target is un-aware till the last moments. Both as a (night) bomber defender and (to less extent) as the interceptor, planes could not fly all the time on full throttle. Fast cruise was the typical practical speed.
The initial idea for the mosquito came from calculations of maximizing range*payload/trip time. This brought de-Havilland to realize that the optimum is achieved (at the available technology) in a twin engine setup and that defensive turrets reduce this so much that they are not worth it.
