The fastest electric bikes around are usually based on POS walmarts or old junkers found at garage sales. Once you move over into electric, its not the bike that goes fast.... its the motor/batteries/controller.
If you arent dealing with lead acid cells you can fit quite a bit of power into the center triangle. In some of the early versions of my bike, I had 72v 20AH worth of batteries mounted in the center. Ive since moved to mounting them forward of center and off the side to better balance with the motor in back. Also im not sure if you've ever tried to carry an 80 pound bike without lifting from the cross tube, its a little on the hard side..... In short, dont limit your design opertunitys, it might turn out that there are better options then you think.