I think the fact I read, and it may have been in Birch Matthew's book was the production run numbers. IIRC production units were not produced by Bell until Nov 1944. Then due to the rate and flow, including Ferrying, squadrons were't converted until April of 1945. the was over a month later.
So it's possible they saw some eastern front action, but that is neither here nor there. They did participate in the attack on Japan and HiTech has acknowledged that they qualify by his rules.
So maybe if we all want it bad enough it will move up the list.
I think the real problem for HiTech with this plane may be the lack of real performance data on it. There is data for the P-63A Prototypes that were evaluated by the army, but none of that data is with Water injection. The base engine was listed as 1325 HP, which is only 15 more than the P-39Q. The Water injected engine supposedly made 1800 HP but there is no real test data showing the plane's measured performance with water injection. But the bulk of the production A-10 version, and the later C model all had water injection. Even Birch Matthews couldn't find any measured performance data on the water injected P-63. That's troubling.
So who would HTC model it? on the test data, or correlate the performance on the prototype plane then add 500 HP? I think that would be the biggest challenge.
That said I'd take the 1325 HP version for the extra wing area 25% more wing. And it was a lower drag wing as well. so it should be a better climbing, better turning P-39 with an extra 28 rounds of 37mm.