I agree with many comments before that it will probably be a while before the substation goes away. The demand isn't high enough for anyone to fork over the money to relocate the substation and I can't imagine the city moving it as a stimulus thing to generate growth in downtown or anything...without one of those two things happening, I don't see it moving. The supply of decent sites in CBD and relatively near districts as well as Domain, etc., is just too high to justify it. That said, if there's a filthy rich fellow who lurks on this site and wants to improve the aesthetics of that area of downtown, "WE WANT YOU TO MAKE SOMETHING HAPPEN THERE."
If there is a significant CVC there, build something for Austin Ballet or another use that would be a blessing to the community at large (perhaps with an emphasis on the arts in some way) and then build a monster tower on the current Austin Ballet lot.