A I said there were huge handicaps in development due to secrecy with the P-59 program which prevented collaboration with the NACA, or even the use of any decent wind tunnels (issues Lockheed didn't have to deal with, granted their designs were better and they had more background in high speed paper projects- L133 jet project). It was also over-engineered to be safe so the designers could be sure that it would at least work.
Look at the early Meteor, it wasn't much better, but unlike the P-59 the Meteor continued to be developed. On the other Hand the P-59 was abandoned as a combat plane soon after the delivery of the YP-59A's and only minor improvements were made to the Production P-59s. Similarly the Meteor could have been abandoned in favor of the Vampire as was done with the P-80. One major advantage the Meteor did have (besides continual development and interest) was low altitude performance, the Mk.I could do 410+ mph at SL and ~420 mph at 10,000 ft and could maintain this unlike piston fighters which had to use brief WEP usages, while the P-59 performed no better than piston a/c down low, though it did out perform most above 30,000 ft and had better maneuverability at altitude with the low wing loading. (although it would have benefited more to use smaller, thinner, wings at the expense of wing loading to have better speed performance:the P-59 having a wing area of 386 ft2 and fairly thick too!!!)
The speed of the YP-59A was 409 mph @ 35,000 ft with 1,650 lbf I-16 engines, the P-59A improoved this to 413 mph at ~30,000 ft with 1,650 lbf J31 engines, the P-59 with slight airframe improvements and 2,000 lbf J31-GE-5 engines managed to do ~450 mph at optimum altitude (~35,000 ft). And the P-59B even managed a max range of 950 mi ith 2x 125 gal drop tanks and increased internal fuel over earlier versions.
The biggest change needed to make the craft combat worthy would be the wings: scaling them down in all dimensions to ~85% would bring area down to ~270-280 ft2 somewhat thinner (the same airfoil but smaller in all dimensions) and span to ~38-40 ft. This would cut down on drag considerably, increase roll rate, and low altitude speed at the expense of higher wing loading- ceiling may actually improve due to the thinner wings and reduction of shock stall. (though wing loading would still be under 50 lb/ft2 at max load, and under 40 lb/ft2 normal loaded, similar to the Vampire I) Other than that small improvements in control surfaces and flaps, a redesign of nacelle/intake and general streamlining, and a better canopy. (armament was easiliy changed, the P-38's armament being probably the most practical) It would never have been as good as the P-80 and not as versatile , but it could have been much better than it turned out imo. (not mostly Bell's fault, just a bad combination of decisions and cercumstances)
The craft also had the highest thrust/weight of any fighter design of the war with 2x 2000 lbf engines (used on the last few P-59A's and all B's) and a max takeoff weight of 12,700 lbs of the P-59A (13,700 max in the P-59B) and 10,800-11,000 lbs normal loaded it had a thrust/weight of .31 to .37 at takeoff! (compared to ~.28-.32 for the early (Y)P-80A, ~.25-.29 for the Meteor I and III, ~25-.27 in the early Vampire I with 2,700 lbf Goblin I, and ~.28-.31 for the Me 262 and He 162 without over-rev, the He 280 had more with 004A and B engines but it was not matched well at all to these-- overall performance dropped despite the increase of ~650-670 kp in late HeS-8/HeS-001 prototypes to ~820-860 kp per engine of the 004 without over-rev of the 004A as they were bulkier -though diameter was about the same- and weighed twice as much and range went to hell as well)
Yes it only saw operational service as a conversion trainer (and later a drone/target towing a/c) And as it was it was fairly well suited for conversion training with good glider characteristics (good for engine outs which were common in testing) and had friendly handling characteristics.