I just saw this and have a couple of thoughts on this subject.
First, the CDo of .0213 posted in Dean's America's 100K for the P-47 is for the XP-47 on page 113. The CDo .0251 he posts on page 598 is for P-47D. The CDo for the F4U-1D is .0267.
Second, those CDo values are for 250 mph at SL. Between RN=2x10^^6 and about 20x10^^6 the CDo values steadily decrease, stabilizing to a constant (but uncorrected for compressibility). Look to pg 112 for the example of P-51D CDo vs RN.
q= Dynamic Pressure = Rho*V^^2 = 160 #/sq ft
At 250mph TAS at SL, RN for P-47D = ~ .00237x250x1.467X(7.29)/(3.7373x10^^-7)= 16.9x10^^6
For the F4U, RN at 250mph at SL =~ .00237x250x1.467x(8.12)/(3.7373x10^^-7) = 18.3x10^^6
For the P-51D, RN = .00237x250x1.467(6.63)/(3.7373x10^^-7) = 15.4x10^^6
Go back to pg 112 and pick off CDo for RN=15.4x10^^6 to find .0176. At 10^^20 the CDo (uncorrected for M) is .0156. At that RN, the P-51D is going about 324mph TAS and about 0.42M
Back to your question "what is going on about relative Drag?"
First the Drag(total) = Drag(parasite) + Drag (induced)
Induced Drag CDi= (CL)^^2/(Pi*AR*e); Assume as Dean states e=.85, P-47 AR=7.29, GW =14411; F4U AR=7.83, GW=13698
P47 CL= W/q*S=13698/160*300 = .285; CDi= (.285)^^2/(3.14*5.61*.85) = .00544
P-47 Total Drag Coefficient at 250mph, SL= .0251+.00544 = .03054
Total Drag47 in pounds= CD*q*S = .03054*160*300 = 1466 # (Dean Table 104 has 1485)
F4U CL = 12694/160*314 = .264; CDi= (.264)^^2/(3.14*5.35*.8) = .00488
F4U Total Drag Coefficient at 250mph, SL= .0267+.00488 = .0315
Total Drag4U in pounds= CD*q*S = .0315*160*314 = 1586 # (Dean Table 104 has 1580)
The F4U with 2000HP at SL at 250mph and 80% efficiency has Thrust=.80*2000*550/367.5
F4U Thrust in # = 2394 # (excluding exhaust) (Dean Table 104 has 2400)
The P-47 with same HP at SL has the same Thrust = 2394 # so the delta excess thrust for the P-47 over the F4U is (2394-1466=928# P-47) vs (2394-1586=808# F4U)
So, for these weights and HP and engine and Oswald efficiencies, relative aspect ratios and wing loadings - the Jug should be faster through the M=.55 range. Above that, you will have to look at CD vs RN and the Compressibility Drag rise curve.
Additionally Dean did not look into the relative CD vs CL Form Drag due to Angle of Attack.
For a P-51D at CL=10208/160*233.2 = .27, the Delta CD is .00018
Hope this helps.