Another thing that has a big influence is the converter. Some converters will allow the rpm to jump to well over 5000 rpm almost immediately. Looking at the mph curve will tell you if it is really transferring power to the tires or is it just slipping.
With a 212 cam, I figure it will still be making power at 5800 or so and I try to shift there because funny things can happen once you pass 6000 even if the car is still accelerating at that point. Also, as the stress on the engine increases by a factor of four instead of linearly with the rpm increase, I don't see the need to try to make the engine do something it was not designed for. Shifting a bit early in 1&2 and let it run out the back door seems wiser to me.
