What if he aggresively deny your fuel point early? Which he can do playing with T0-T1 with no tech. If you dont control fuel at least for a short time - there will be no 7-8 min window - so those T3s will be useless.
With a con-max-con-max-engy opening you will have no issue holding your own fuel, and you don't even need to hold more than half the map for the first 7 minutes of the game. You seem to severely underestimate the window of soviet T3. It's not some 2 minute window that you have to exploit to the maximum potential in order to gain some benefit from it. If rush a T70 (T2->T3->T70) while holding half the map, you will not be facing P4s for a long, long time. You can in fact, build another T3 structure and get another T70 and that will come out at the same time as the first enemy P4. That is how big your window is.
Just look at the fuel thresholds (in other words, the amount of fuel you need to collect to afford a unit when including tech cost):
M5 quad rush: 110 fuel (80+30)
T70 rush: 150 fuel (80+70)
P4: 275 fuel ( for T1,T3,P4) -> in practical terms, you NEED T2 with ostheer or a light vehicle rush will simply destroy you, so it's 295 fuel in the actual game.
So assuming you have had roughly 50% of the map, your M5 quad will hit ~1.5 minutes before your opponent even has
half the fuel he needs for a P4. The window is huge! If you are mildly successful with your first quad (which is not hard), you expand the window further and you can easily fill it with more light vehicles (like getting a T70 and another Quad). At this point you have so many light vehicles on the field that it is impossible for you not to get huge map-control and overrun your opponent.
As I have said in another thread; this is why mobile defense is now suddenly so popular. It does away with the need to tech up for Ostheer so they actually stand a chance against the light vehicle onslaught. The only way I have been able to stand up to this without mobile defence is when my opponent plays too greedy and fails to get minesweepers.
Unfortunately the mini-update wiped the replays I had against Ostheer so you have to take my word for it that it works. But you can't argue with the simple math; a P4 will never come early to punish your T3 rush, it can not happen.