what is this talk of now you have to have less units if you want to build panther/ brumbar? the difference is 160 manpower, last I checked no unit in the game cost less then 200 and that is just pio's. In comparison the fuel was dropped by 35 and 30 giving you about a minute faster tank, fuel wise (assuming you have at least half the map that is). Sure is it going to take you slightly longer to save up the manpower, yes.
If your saving for t4 as a competitive build vs. tight builds and good players you need fuel way more then manpower and the the extra manpower you pay is less then what you should be bringing in in a minute. Does this make panthers viable in 1v1 vs. good players with good builds probably not but it puts it more in the right direction I think.
You're forgetting about reinforcement costs for existing infantry. Every additional second I have to wait for floating 600 manpower is another second that my wounded squad does not reinforce and just sits at base.
It's not too bad for the first Panther/Brummbar if you're rushing it, but it becomes a huge problem late game since you are never going to be floating 6-700 manpower in a close game.