I’ve only played as the allies in my one game, but my opponent waited for G4 to attack and it has not turned out well. A southern Soviet counter offensive has put the Balkans into allied hands, but the supply line is long and both Italy and Germany are busing back. I think if Italy can get a few tanks and mechs in Syria that they could do a lot of damage in the middle east or southern/eastern Russia.
I think Italy and Germany attacking together would be best for the axis.