I ride a Lightspeed Ultimate, which is 6/4 in the bottom bracket area and 3/2.5 in all other places.
I can definitely tell a difference between the 6/4 in the bottom bracket area and the relative flex of 3/2 in the Lightspeed Classic. I really felt the difference when trying both models with comparable wheels, tires, psi, seatpost, etc.
I am 185lbs and have seen and heard the same thing from others.
(i am cat5)