In most cases these are unnecessary. If you are using a plane from 1945 as your selection it is already using one of the biggest, baddest engines of WW II, there is nothing to upgrade to. It will also being using the latest (or rather last) weapons fit of WW II and will have nothing to upgrade to. a 1945 selection will also be equipped with a 1945 gunsight and not need upgrading.
Using the Tempest II, where do you go from a 2500hp Centaurus engine? upgrade to what? It has four MK V Hispanos with 750rpm cycle rate. 50 rounds a second, 1/2 the rate of Vulcan gun, What are you going to upgrade to?
P-47N already has a 2800hp engine, upgrade to what?
Are you sure in most cases unnecessary? Sometimes a few minor superior attributes add up over time to a very large advantage. The Tempest you mentioned was after all originally equipped with " one of the biggest, baddest engines of WW II" and it was upgraded to a completely different engine.
As far as "the latest (or rather last) weapons fit of WW II", remember this is a scenario where you are limited to one fighter aircraft type for all missions. Perhaps efficient accomplishment of a mission would require an upgrade for example to 20mm or 30mm cannon instead of the eight .50s in a P-47 or .50cals for a Tempest II. The capability and ease of doing could minor detail for the Criteria. If your primary opponent on a mission outnumbers you with relatively easy to shoot down aircraft, more .50cal bullets would be preferable to less 20mm shells. Perhaps for some missions an upgrade to a single really large cannon for each wing would be advantages. How well can your fighter's wing handle it? Even though the history after WW2 quickly decided that cannon rather than .50s was the way to go, in WW2 it may or not be depending on the mission. The improved post WW2 fire control systems are the primary reason for it now being much easier to hit with less ammunition expenditure.
A P-47N may have benefited from additional equipment to provide even more horsepower for low altitude performance comparable to the Tempest II and the same could be said for Tempest II performance at high altitude, if the airframes can take it.
Are we really sure that all gunsights in 1945 are equal, couldn't one manufacture/nation have sight have small advantages that add up over time?
You mentioned in an early post that the Centaurus engine had a great career after the war with long overhaul intervals, what about after battle damage occurs, does a sleeve valve engine hold up as well as a conventional radial? Perhaps it doesn't and you decide to change to a R-2800 like most air racers have out of necessity due to parts I assume. We already know this "upgrade" is possible in the Sea Fury, one of the planes that could be a contender in this thread.
Awhile back someone mentioned Boyd. I've got Coram's "Boyd". No doubt Boyd was very astute about small light-weight fighters and energy management. That being said when you are limited to only one type of fighter, as you are in this scenario; light, fast and maneuverable up/down/sideways is not enough. You have to do most things well enough, and small light-weight fighters (A6M, P-39, Yak-3, etc.) do not do this.
Been following this thread and decided to opt for the Corsair F4U without naval equipment it would sure help out performance, also like the US radial engine and the aircrafts allround capabilitiy it sure must be in the top 10 for every catagory you can think of
Thanks for participating. Some participants certainly agree with you. No doubt it would be in the top 10, and more likely the top 5.