The reason why i will not do that? Because it would invalidate a huge part of GT, namely all transforming beween different voltage tiers... If higher tiers would be the same or better efficiency, you would only build the highest tier machines and remove the low tier ones. The whole base would become one, at best two voltage tier/s and that would be simply booring...
Devil's advocate: fwiw, even Greg's thinking twice about this :p
It doesn't really make a lot of sense to keep ancient technology kicking around in your base just for diversity. If I make a better toaster, I should just recycle the old toaster's parts and move on.
Our mission, if we choose to accept it, is to ensure even with just a couple tiers that its NOT boring!