Matt,
Cast iron/aluminium pistons do have a piston speed related argument for choice. However, my 4 3/4" bore by 6 1/4" stroke tractor engine running at 1125 rpm at max power would indicate that CI is good for quite a few rpm above 650. Can't remember, and am not looking for, the generally accepted cut-off point but suffice it to say that there were probably other reasons for changing to aluminium pistons. Reciprocating weight and minimal crankpin lubrication come to mind along with cost, cost, cost. CI was probably only kept for replacements due to minor changes in balance or problems of running aluminium pistons in Listard cylinders (if there is an issue).
Regards, RAB