Actually, mech inf and heavy artellery works GREAT together.
What useually happens is that you start of with a few artellery and many infanteri.
As time goes by, you sacrifice your infanteri, and at the front you get less and less infanteri and more and more artellery. With heavy artellery you should be able to bring fresh troops to the front faster, thus easer using the heavy artellery benefit.
When that is said, mech inf or heavy artellery are far from gamebreaking.
Do anybody think that heavy-armor (tank gets +1 defence) be overpowered?
Another question, does improved production have a second hidden benefit? Obviusly the +2 units helps a lot if placed in a 3-income or 2 income province, but is it also a good benefit to be able to build 12 instead of 10 units for germany?