There was no 'very advanced fighters' by Japanese in Dec 1941, and, if anything, Allies were a tad underestimating Japanese gear, not over-estimating.
In Dec 1941 the Allies were caught by surprise, The Japanese had more advanced aircraft than the Allies suspected. They also didn't know that the Japanese fighters were using single speed superchargers.
By the time P-47 flew, they (Americans) also knew that it was the fastest thing in the skies, and three factories were contracted with making them, thus adding quantity to quality.
The XP-47B flew in May 6th 1941,
There are 1583 on order on Oct 14th 1941
1st P-47B comes of the line Dec 21st 1941......but changes are needed
Jan 1942, 1050 P-47Ds are ordered from Evansville and 354 are ordered from Curtiss. 2987 are on order.
March 1942 5 P-47s are completed, production stopped to switch to metal ailerons.
June 1942 (Midway) The 56th fighter group receives a few P-47Bs. They lose one in flames. They run into compressibility problems. they have several fatal accidents.
Sept 14th 1942, 5 weeks after Guadalcanal starts, first P-47C-1 completed at Farmington.
Sept 19th, 1942, 1st P-47D completed at Evansville.
Oct 1942, Curtiss completes first P-47G. which is not quite the same as actual production?
Oct 1942, 3 P-47C-1s are sent to Eglin Field for comparison tests against the P-38F, P-39D, P-40F and Allison Mustang Oct 27th to Nov 26th.
Nov 1942, 56th fighter group is declared operational. The Allies have invaded NA in operation Torch.
Dec 20th 1942, First P-47Cs show up in Liverpool as deck cargo.
Dec 31st 1942. 532 P-47s have been completed, 10 of them by Evansville and 6 by Curtiss.
The quantity was taking a while to show up. In Feb 1943 Gen Kenney is told no more P-38s, He requests P-47s. It is not until May that he is told he will get them. They show up June 1943.
There was no threat to the US mainland by the enemy bombers - since US was unable to cover even half of Atlantic or Pacific with their bombers, expecting that Axis does it to the USA was as far-fetched as it goes.
No threat to the mainland but threats to non mainland areas was pretty significant.
If the Merlin 20 in the P-40 will not cut it, stuff it in the XP-51 and see how that goes.
Why?
The US doesn't order any Mustangs of their own (except the two evaluation planes) until April of 1942 (A-36s). The Contract for the P-51A (1200 planes) with Allison engines is not signed until June 23rd 1942. The last Mustang I (British) comes off the line in July, Production switches to the Mustang II but the US swipes 57 of them. July 25th 1942 NA is given a contract to convert 2 of the lend lease P-51s (Mustang IIs) to Packard Merlins with 2 stage supercharges. It is Sept when the first A-36 (US owned Mustang) comes off the Production line. How fast are you going to get a Merlin 20 powered Mustang? Any and all Merlin 20 powered Mustangs built in 1942 and early 1943 will take engines from P-40F and L aircraft. Probably P-40Ls by the time testing and production can be set up and such work may be at the cost of the two stage Merlin program since they have the Prototype flying in at the end of Nov. 1942.
Japanese (especially IJN) were slow in installing the big radials on their fighters, despite manufacturing them.
Very true but while we know that now, what did we know then?
The Ki-44 underwent combat trials in SE Asia in Dec 1941. The Allies did not know how long it would take to get them into production. The Allies in 1942 did not know how long it would take for the IJA to put a two speed supercharger into the Ki-43.