It seems to me that the P-61 was basically a very good design, but one that had been unnecessarily handicapped by a severe case of 'committee-itus'...
The extra weight and drag penalty imposed by the essentially redundant quad.50 top turret and the cavernous fuselage design does not seem to be justified in regards to enhancing the a/c's operational efficiency. A two-man crew tandemly seated under a bubble canopy (ala the F-15 Reporter variant) would have made for a faster, better climbing, and OA more efficient offensive NF, IMO.
Erich's comments regarding the Me 262 NF variants are very interesting. From what I've read of WWII NF operations, it seems that while a a slow and stealthy approach to the target a/c was the preferred method of attack, it was also quite common for targets to appear quite suddenly, thus requiring that the attacking a/c reduce its speed as rapidly as possible so as to manouever into a viable firing position, or at least avoid overshooting and alerting the target a/c to its presence.
With this in mind, was the poor response to rapid throttle changes of the early jet engines, and the very clean aerodynamics (Even with the draggy 'antler horn' radar antennae, I'm presuming that the 262 was still significantly 'cleaner' than equivalently equipped prop NF's) considered to be a significant problem? And was there any effort by the designers to develop an equivalent to the 'speed brakes' of early post-war jets?
JL