The factor you are talking
The factor you are talking about is the charge rate a battery pack can safely withstand. That the 85 kWh pack can withstand a 90 kw charge rate, suggests the cells can safely withstand a 1C charge rate. The 60 kWh pack at a 1C charge rate would require the SuperCharger to tone down to a 60 kw charge rate, and provide fewer miles per hour of charging.
Good point.
But Tesla hasn't released the figures required to determine whether these suppositions are true.
Some cells can take greater than 1C charge rate, FWIW.