The XP-82 performance closely matched the P-51D, except it was faster and climber better at same RPM/Boost.
At 20000 pounds combat weight full internal fuel (575 gallons) the W/L was about 47. The 1650-11/21 developed 1405 Hp at 25000 feet, but capable of 1780 Hp at 22500 feet at 90" and 1860 hp at 19,400 at 90" and 2270 Hp at 4000 feet at 90"
By contrast the P-38L with full internal fuel 410 gallons, full combat load was 17,700 pounds and engines at 60" (Max Combat Power) at 26500 feet generated 1600 hp.
At max combat power the comparisons between the XP-82 (with Merlin engines) and P-38l (or K) were
P82 WL = 47, Hp/GW@ Max Combat Power = 2x2270/20000@90"= 0.277, carries 575 gallons of internal fuel. The P-82 HP and max Combat power at 22500 feet = 2x1780/20000 @90" = 0.178
P38L WL= 54, Hp/GW@Max Combat Power = 2x1600/17700@60"= 0.181, carries 410 gallons of fuel. The P-38L HP and max Combat Power at 26500 feet=2x1600 Hp/17700@60" = 0.181
The engine/boom mass combination for the P-82 is a smaller distance from Center of gravity than the twin boom P-38, has no center gondola and a shorter wing span (51 vs 54 feet). It should roll as well or better than the P-38 and accelerate, turn and climb better. It will certainly Dive better.
I have not seen maneuverability related data but the wing loading and Power ratios strongly suggest a significant advantage in all aspects from Drag and overall agility plus guaranteed speed and climb advantage based on the hard data.
The P-82 was much faster