I'd rather reduce the cost of the panther and brumbar. But Your option would also make sense - I just feel that ppl seeing no fuel for something Wehr gets might go really wild.
Current teching situation:
T3 = 90 + 15 = 105 fuel
T4 = 90 + 25 + 25 = 140 fuel
T3 building into T4 = 90 + 15 + 25 + 25 =
155 fuel
New teching situation:
T3 = 105 + 0 = 105 fuel
T4 = 105 + 35 + 0 = 140 fuel
T3 building into T4 = 105 + 0 + 35 + 0 =
140 fuel (but of course more manpower cost for teching)
You already pay a pretty steep price premium for T4 units, so imo you shouldn't need to pay a huge tech cost on top of that. Consider the first Panther after constructing T3 costs 690 manpower and 235 fuel initially with no side benefits for the tech.
T4 is balanced in team games, but underpowered in 1v1. This would be a safe solution.