I know my springs are crap and I do need to change them next year (hopefully), but I find my car handles really well with the 7/8" bar I have on it now. I would have no problem with that diameter if it fitted better at the frame mount points and lined up better with the shock mounts. Just my opinion, but as the current bar is possibly made to (sort of) fit a few different vehicles, I might still trust their engineering knowledge as to what is best for a Mustang with comp suspension. I would agree, a standard suspension car might only need a 5/8" bar should an owner wish to fit one.
And yes, it would be very nice to be offered some sort of deal to Forum members. No, I'm NOT holding my breath on this one either.