My theory shoulda been qualified with a great big "In general" but yeah internals will definately make differences in reliability/safety in the upper rpm ranges. I just kinda tossed it out there for discussion.
Personally, I've spun the same cast crank to 7300 rpm reliably for 3 seasons straight but definately dont/wont recommend the use of em up there for obvious reasons. The availabilty of all the cheap aftermarket cranks nowdays dont justify the danger messing with stuff like that.
Please dont anyone attempt this 7300 stuff based on what I got away with years ago. It just too damned easy to drive in your own oil and hurt yourself or the other guy in the other lane.