OK, here is a quick numeric review of the options…
On J1, all are pretty much equal… at most 2 units to Asia (assuming SZ59 was attacked).
With 1 IC, 2 TRN purchased on J1, you can get an additional 9 divisions to Asia on J2 (1 existing TRN, 2 purchased, 3 units to be built at the new IC)
To do this, you will strip Japan, Oki, and Wake. You also buy 1 more TRN in J2
With 4 TRN on J1, you can get an additional 10 units to Asia on J2… in theory since you have 5 TRN now. But where to get the units?
Japan has 4 units left after J1.
Oki and Wake have 1 each.
Phillipines has 2.
Nothing else in range for immediate landing.
So only 4 TRN full, one is sent sailing away for no gain this round.
Only EIGHT units to Asia (one less than with the IC).
You can buy AT MOST 8 units due to build limits
With the original IC buy, you added a 4th TRN the previous round, and now you are landing 8 divisions from Japan, 3 more placed at your new IC for 11 total.
With the 5 TRN buy, you move the 8 divisions you bought for Japan in J2, and you MAY land 2 units from the TRN sent scavenging the previous round (maybe not if it went to Solomons and New Guinea). 10 divisions total.
Each turn after J1, the IC in Asia gets one addiitional unit to Asia. And by the end of J4 or 5, the extra TRN is wasted, because there are no units left to scavenge.
Also, to buy 4 TRN on J1 requires a 2 IPC bid to asia, so less income for Germany is also in the mix.
And even if you build an IC on J2 or later, you are still down 1-2 units in Asia. May not sound like much, but in a hard fought game against a skilled opponent, a single INF in the right place at the right time (like an extra INF in the front wall of the Japan attack that you get with the IC), you are able to take territory stornger and faster, and do more damage to your opponent with less loses of your own forces. And extra INF to Asia by Japan early has a cumulative benefit over several turns.