How fast can you go with a Magnum? Who has the quickest setup?

I think you are right on the biggest limitation being head flow.
Engine quest says it's pointless to do anything beyond a cleanup and smooth of their Mag heads due to them already being about as high flowing as possible right out of the box. (better than OTTB Edlebrocks they say)
The EQ head with the 1.92 valves and .512 lift flow right about 230cfm, and with a few bolt on's the 5.9 can easily go over 400hp with a stock bottom end.
Mine was rated at about 420 with the cam EQ heads 1 5/8 long tube headers and 1.92 valves.