@variant:
Germany’s and Japan’s fighters should attack at 4 (and maybe defend at 5) to represent the Mitsubishi A6M Zero and Focke-Wulf Fw 190.
Given the poor armament of the Japanese Zero, two 7.7mm machine guns and two slow-firing 20mm cannon, attacking at 2 might be more accurate. As for the Focke-Wulf, with an armament of two 13mm machine guns, two 30mm and two 20mm cannon, plus in some variants, four 20mm cannon in underwing pods, they did shoot down Allied bombers on 17% of their passes, US Strategic Bombing Survey figures, but were also quite unmanueverable and needed the cover of the ME-109s to protect them against fighter escorts. However, that is more suited to a tactical game like Guadalcanal than a strategic game.
American Battleships and Aircraft Carriers should defend against air attack at 5 if you wish to be more accurate historically. Allied destroyers should after two or three turns in a game attack submarines on a 4 to reflect the use of the Hedgehog and Squid ahead-thrown ASW weapons. Allied patrol bombers should be able to attack submarines without a destroyer being present to reflect the development and use of sonobouys and air-dropped homing torpedoes and Magnetic Anomaly Detectors. Japanese ships should defend at 1 against air attack given their very poor antiaircraft performance and batteries. Aside from the Kaga and Akagi, carriers should defend against surface attack at 2, more like one of the A&A50 destroyers. Then there is the use of incendiary bombs, roughly 5 times more effective than HE bombs against burnable targets, like Japan. You have massive improvement in air to ground weaponry with the use of rockets, napalm, phosphorus bombs, massive machine gun batteries and airborne cannon on the B-25 Mitchell, and the use of parafrag and parademo bombs. The Germans and the US both developed and used radio-controlled bombs against pinpoint targets.
There are all sorts of things that you could add to the game. Just depends on how far that you want to go. Not sure about the US transports for 6 IPC though. One IPC, one million man hours buys you 5 Liberty or Victory ships, so 8 IPC would buy you 40. Given that the US basically built enough merchant ships in WW2 to totally replace all losses, as a rough guess, need to look this up for more accurate data, say 6000 ships, that would equate to about 1200 IPC, 8 X 150. Assuming the US gets 45 IPC average, that would be close to 30 turns of nothing but transports being built. Since the US did build a lot of other equipment, might be easier just to give the US and automatic build of two transports per turn like in Xeno’s Pacific at War game. Actually, you would need to give the US 4 transports per turn to reflect the East and the West Coast.