I beef up the mech forces over the winter of 1941. I think during Barbarossa it is more cost effective to have lots and lots and lots of infantry.
My pre Barbarossa build order does allow for about a dozen tank/mech corps. The disbanded mech corps are reformed as 35% experience units. This results in fewer of them, but they are twice as strong. With 2 tech advances, they end up being 8 point units and can hold the line. The Soviets start with 3 armor corps at 30% and these I judge to be reasonably effective and don't disband those.
In 1941 given the low mobility of these units, they can do very little more than hold the line and perhaps occasionally counterattack. I like to put them in places like the Smolensk corridor to cover the clear terrain. That's a good choke point for them.
20% experience mech corps are junk and as a German player I make it a point to chase those down and kill them outright if they appear on the front line -- they are not going to survive to get the experience. This is much harder to do when they are at 35%.
I'd much rather harvest the production from those trash mech corps and reform them into more effective units, even if there are fewer of them.
You really don't need more than about 15 mobile corps as the Soviets anyways. More than that is nice, but not essential. What *is* essential is infantry and lots of it. A huge mass of pre war rifle corps can cover the front and buy time for the rifle armies to show up, and also make use of Soviet manpower and helps avoid manpower problems down the line. Manpower constraints will cripple the Red Army, and the game mechanics are such that either you use this prewar manpower or lose it, it doesn't stack past the cap. So the rifle corps hold extra manpower as buckets.
I've put a lot of thought into this, believe me. Getting the Red Army right is the hardest thing to do in this game. The other powers are much simpler to manage. I have satisfactory build orders for the rest of them, but Red Army has always given me problems until these rifle corps got fixed up.
Flavius, I would like to pick your brain some more if I may. With the 20% infantry corps do you turn repair/upgrade off so that production isn't wasted on doomed/rather useless units, or do you feel that they need to be upgraded to be of any use at all? Also do you remobilize them prior to Barbarossa, or leave them to die in place as garrsions.