Since nobody ever built a metal Mosquito we don't know what the weight difference is. We do know there are different forms of wooden construction. We do know that a wooden Japanese KI84 was about 600lbs heavier than the Aluminium one, we also know that a version using more steel was also heavier. We know that Russian planes also got lighter (in structural weight, they used the weight savings to add things) as they got more metal parts. We know the wooden "tail" on the 109 was heavier and required a counter balance. We Know the Americans had trouble with several wooden aircraft projects coming out over weight.
We also know the P-38 was
designed for a 12 G ultimate load factor with a an 8 G service load factor (later versions may have slipped a little?) while the Mosquito was 8 G ultimate and a bit over 6 Gs service load. Not a criticism of the Mosquito, it wasn't designed as a single seat day fighter and it was designed to British rules, not American. I would not that many post war US jets had service G factors lower than 8 Gs.
The Americans also had some sophisticated wooden construction techniques, perhaps they weren't as good as the Mosquito but without real engineering data and a structural engineer or two we don't know if they are equivalent. Some US methods used a lot of resins and baking entire fuselages in large ovens. It may not have been true but they were advertised as rot proof
They were reported to be subject to decomposition over time but 10 out of 262 built were still on the national register in 2001, 58 years after last built.
I could be wrong but part of the Mosquito's strength/weight could be due to it's size. As in a 4ft diameter tube is stronger (at least in bending) than a 3ft diameter tube if both use the same wall thickness. How much thicker the 3ft tube walls have to be to make it as strong I don't know. If they have to be 33% thicker than the wight comes out even.
The Mosquito was a much larger airplane and trying to build the P-38, especially those skinny tail booms, out of the same materials as the Mosquito might have been difficult.
The Hornet did use slightly thinner fuselage skinning than the Mosquito. While the inner and out layers of plywood stayed the same the inner layer of balsa wood was changed from 7/16 in to 5/16 in in thickness. Please note that balsa wood can weigh, in general (model airplane contest grades excepted) between 8-14lbs per cubic ft and picking 12lbs for ease of math means that q 1 in thickness 1 sq ft in size weighs 1lb. the 1/8 in difference between the Hornet and the Mosquito is worth 1/8 of a pound or 2 ounces per square ft. Using lighter balsa the difference is less.
good website on the construction of the Hornet;
fuselage construction
Parent :
The de Havilland "Hornet" & "Sea Hornet"
The Hornet was also much smaller than a Mosquito,
with 9 ft less wingspan, 80% of the wing area, around 5-6 ft less fuselage length ( depends a lot on radar domes, which can add another few feet to either) and the Early Hornet weighs about 3,000lbs less empty than a Late model Mosquito (both with two stage engines), refer to the above website for the use of a partial metal wing spar.