Doesnt an un-upgraded Ober loose vs most allied infantry when it would come out, IE vs a squad that has a weapon upgrade and 2-3 stars of veterency?
I mean its very plausible that an ober at that point is facing a vet 3 penal with 6 men and .85 target size which has an overall more base long range DPS than an ober.
If your opponent has vet 3 mainline infantry by the time you rushed a 95 fuel (15+80) construction right after medic HQ (= 25+15 = 40), you are doing something wrong I am afraid.
You usually never have less than +20 fuel income at average, so its easily possible to rush the unupgraded t4 at the 6 or 7 min mark. (medic hq into t4 without medics or flak HT = 135 fuel)
In 2v2 and upwards that is. In 1v1 its gonna be a little bit slower