Not fantasy numbers at all, there is plenty of test data to back it up. And yes, the P61, for what it was built for, was a good aircraft, but certainly not the equal, or even a close runner up, to the P38L in it's final incarnation.
Incidentally, there was more than 1 variant of the 38L. The 1600HP version (actually basically the same P38, but not given the Ok to run at a higher supercharger boost setting) had performance numbers slightly lower, 425 MPH@25,000, max ROC of 4100 FPM to 5k with a climb to 20K of 5 min, etc. All in all, still far superior to the P61.
You might also be interested in knowing, the 38L was equipped with a tail warning radar system.