I have not tried IRONMAN on Japanese side, but on third day as Allies, my bombers accidentally attacked Yamato with RUFEs on CAP north of Miri, and around the same time LEXINGTON met two CVEs south of Wake (and sunk one of them - not that unarmored CVEs have any chance against fleet CV), so historical composition of those Scenarios is quite "free".
I would actually recommend my own MOD (and I need people to better test it ). It was specifically made to work with current AI. And if you get Scenario 31, with current BETA patch, you would see Japanese planes with profiled bombloads (different bombs for different missions - you would LOVE BETTYs with 800kg bombs!).
There is general reduction of ship capacities, and enlargements of support devices, so all sides need more ships to haul their troops. Japan have also slower research, and less manpower, and fuel is being produced. SNLF squads are weaker.
Allies have early deliveries of few hundred older fighter models, so beginning should be definitely tougher for Japan.