I just did a quick plug-in of data for the P-38J. It seems that the P-38J should have higher turning rate than the F4U both with and without flaps (unless I made an error, of course) but a larger turning radius.
I was using, from America's Hundred Thousand:
A = 8.24
S = 327.5 ft^2
BHP = 1600 HP
N = 3000/2.1 = 1429 RPM
D = 11.5 ft
W = 15,500 lbs
From P-38J pilot's manual at W = 15,000 lbs, clean, v_stall = 94 mph ias = 110 mph CAS. With full flaps, v_stall = 69 mph IAS = 84 mph CAS. So, C_L_max at 15,500 lbs = 1.5 flaps up and 2.58 with full flaps.
I assumed e = 0.8 for clean and e = 0.9 with full flaps. I assumed C_D_min = 0.02 clean and 0.09 with full flaps.
Doing all this in the spreadsheet (a link to which is in the my document), I have to correct for T = 2 * (thrust per engine), and I get:
max turn rate with flaps = 22.75 deg/sec, radius = 525 ft
max turn rate flaps up = 20 deg/sec, radius = 875 ft
The spreadsheet I'm using is here:
http://www.electraforge.com/brooke/flightsims/aces_high/stallSpeedMath/turningPerformance-v3.xlsNote that for P-38's, you have to manually modify the T column (add a multiplier of 2 in the formula).