Was it up to me, it would be T3 unlock with side cost of 20-30 fuel associated.
Good thing nothing is up to you.
Doesn't need that kind of nerf. T3 would be optimal, but T4 at least contributes other things and its built in most games. A T3 sidetech of 20-30 FU would not dramatically increase timing (~1-2 mins assuming equal fuel control and no caches) all whilst simultaneously delaying T4. What a terrible and unnecessary idea.