Testing on 24 May 1943 of the XP-63A revealed a max climb rate of 3,670 feet per minute (fpm) at sea level, 3000 rpm. and 60.5 inches of manifold pressure. It was still climbing at 2200 fpm at 25,000 feet and manifold pressure was down to 45.5 inches Mercury. Absolute ceiling was 41,200 feet.
Testing on 6 Aug 1945 of a production P-63A-9 revealed 3650 fpm at 3000 rpm and wide open throttle from S. L. to about 8000 feet, tapering to 2640 fpm at 20000 feet (still 3000 rpm and wide open throttle. Service ceiling was 41,400 feet.
The P-63E-1 attained at max rate of climb of 4600 fpm down low, had a service ceiling up around 41,000 feet and reached a max true speed of about 445 mph at about 26,000 feet. At 26,000 feet it weas still able to climb at 2200 fpm at 3000 rpm and wide open throttle.
The XP-63A used the V1710-93 and the P-63E-1 used the V-1710-109.
The main issue with the P-63 was stability. In tests, pilots attempting to pull 5 - 7 g in tight turns wound up pulling 10 g and wrinkling the wings. They tried a +1° change in horizontal tailplane incidence and a bob weight, both of which helped, but the aircraft still had very low stick forcce per g. They also highly recommended going from fabric to metal covered elevators to counter the stability issue and that seemed to work better than anything else, though it remaing relatively easy to overstress the aircraft. This appears to be the princliple reason why the USAAF didn't order more of them than they did.
These data come from WWIIaircraftperformance.org .
Other testing I have seen shows the P-63 did better than these numbers when they cleared it for 75 inches of manifold pressure and went to 3200 rpm.
The aircraft itself performed quite well, rolled faster than most other active USAAF fighters, and had a service ceiling above 41,000 feet. The cannon was not particularly liked due to occasional jams, but the cannon was the one supplied by the government; it was not selected by Bell Aircraft.
So it was a little too easy to pull heavy g and the cannon was not liked. The Russians simply threw out the cannon and installed their own. They liked the P-63.