I always build a factory in Manchuria on turn 1 when playing Japan.
In my opinion, using transports to transfer land units to the mainland turn after turn after turn is a complete pain in the backside, because the transports are highly vulnerable to air or sea attack. In order to keep both transports afloat I find I need to hold back at least two destroyers to escort them everywhere they go and if allied air power is nearby, then sometimes a cruiser too.
Directing warships to escort transports is something Japan cannot afford to do, if it is to have any chance of matching or restricting the US navy in the Pacific.
The way I see it, the 15 IPCs a new factory costs is a small price to pay to free up the entire Japanese Navy to focus on blocking the US Navy virtually from the start of the game and until US production eventually overwhelms it (by which time Russia is likely to have fallen anyway).
But other more experienced players may have worked out a better way to put pressure on eastern Russia and hold the western Pacific for as long as possible.