Agree. I need to do a measurement of the resistance of the frame someday. Yes steel is a poor conductor compared to copper but with all that area in the frame it may not be aiding you much to install a copper ground buss. The buss does make less voltage drops if you rewire your existing circuits to it. It makes it easier than using the frame as the buss.
All I need to do is measure the voltage drop from the front frame terminal to the rear frame terminal (TAB) with the fans running. Then knowing the current draw on the fans I can compute the resistance of the frame. Then I can compute the resistance of a #4 AWG copper wire and see what that in parallel will buy you.