Just use canTrain callback and allow obsolete units to be trainable.
To me, AI is more important than performance.
First of all, python is not as taxing as you made it sound to be.
Secondly, look at CiV.
It performs well, nice graphics, not laggy etc.
But it is crap because the AI is total crap.
So you rather have a mod where the AI is crap but performance wise is good, or you rather have a mod where AI is still good but just one or two additional python codes?