Remember, the fastest cb350 out here at PIR has well over 15k put into it to acheive that, though I think mid 40's would be more than reasonable on much, much less money.
I belive the 'slower' ones are running with headwork, special intake manifolds, different carbs, velocity stacks, high compression pistons, aftermarket cam chain and tensioner, hi-po cam, custom exhaust, larger intake valves, and I'd imagine they have to run aftermarket conrods to achieve the reliable 13 to 13.5k redline they run with. With all of that, I'm sure they've probably gone ahead and balanced, lightened and blueprinted the crank, flywheel, etc. They also run with a different 5th gear.