^ If you start with the premise that you get any energy taken up by frame flex returned to you when the frame rebounds, you're not going to be any faster by making the frame more rigid.

As long as the bike is rigid enough carve the line you set it on, and to feel like it's responding crisplly when you accelerate, there isn't much, if any performance advantage to making it stiffer.

Making it stiffer past a certain point is going to beat up the rider, and fatigue them, decreasing performance.

Modern Carbon fiber racing bikes could be a lot stiffer than they are. Take for example the Cervelo R3SL. While the front end and the bottom bracket are designed to be quite stiff, the seat stays are as thin as a pencil, and are designed to flex. Riders have won Spring Classics as well as the Tour de France on that bike. If there was a substantial advantage to being absolutely rigid, Cervelo would beef up the seat stays and eliminate the flex.

So I think its still a matter of determing how much flex, under what loads, and in what direction, is optimal for the perfomance criteria you're trying to meet, rather than just trying to make a bike that is stiff as possible.