I think the brum is going to be problematic to balance with ever increasing accessibility. The easier t4 gets the more lack luster the brum will need to be for potentially come a minute after a p4 would.
That is a also a problem since more powerful unit seem to become accessible earlier in the game.
The tech changes only make a transition from T3 into T4 cheaper. Rushing T4 (rushing a Brummbar) will cost exactly the same as it does now (90+25+25 = 105+35 = 140). The Brummbar does not become accessible any earlier.