What transmission line do you know that has not been designed to run at a particular voltage? "Bumping up" the voltage would still require a revision of the whole line including endpoint transformers etc. The copper cost is not the main expense, the main expense is doing the work of replacing, or doubling up the entire transmission line. Also, the voltage increase would need to be proportional to the change in load. Reducing charge time from 30 minutes to ten suggest a triple fold increase in capacity. A 400Kv line then would need to go to 1.2 MV. That would be a pretty special line, very expensive :)