I think the Axis would need the equivalent of a whole extra turn, or about 40 bonus IPCs anyway. This would make it super easy to secure the Mediterranean and the Middle East, gaining a pernicious IPC boost vs. the Allies, and eliminating any southern threat altogether – then you could safely marshal an amphibious force out of Allied striking range. It would make for one heck of a battle from the North Sea to the South Atlantic!
Does anyone ever play by the rule that to win, the Allies must hold all four capital territories for one full round? (The manual just stipulates that the Allied conqueror of Germany must hold his own home capital – in other words, the Yanks or the Brits could hold Germany, while Moscow is occupied, and the Allies still win.) I guess in most cases the hold-all-capitals rule might just extend the game while every German unit is hunted down across Russia – but there might be an extreme case where the Axis player could retake Germany while holding Moscow – very tough when your reinforcement income is zilch.