I donīt really see the point in your suggestion. The startup bonus provides more than enough construction units to build some factories and produce ships. Before that, the 72 ticks protection guaranteed enough time to do the same.
Even worse, Iīd expect a change like this to make attacking even harder. Basically you could concentrate on mines and FCs, generate a large stockpile with very few factories and just stock up on the number of factories shortly if you expect/get incomings.
Also, as you pointed out, this would most likely favour large planets due to increased costs for speeding up constructions.
Iīm pretty sure you could have noticed these flaws yourself, so next time you get an idea in the bathroom, think it through while enjoying a coffee afterwards
(Hope this all didnīt sound like flaming, it certainly wasnīt intended to be a flame!)