Heres a screenshot of my game... this thing has been over since I declared war on poland. Ive been at war with the SU for less than 2 weeks and they are pretty much run over at this point. Note my practicals and IC- also, due to manpower issues I havent built anything but naval units for a while... so they have actual decayed quite a bit.
Wow. I'm amazed. I've never gotten this far into the game, but 900 IC? Wow. And look at that leadership and those practicals. Very nice.
So, I suppose one lesson (for spamming) is to build a ton of parallel. While it won't be efficient for those units as doing one at a time, after that is ton you'll have so much practical that building subsequent units will be very cheap. It would be interesting to see the numbers behind how this works, but your practical example seems to make it quite clear!