Yes, the tiers would not entirely be based on the highest firepower number. There are several things weighing in like burden and number of boxes. Higher burden do give a higher firepower for a longer time while being hit.

That's one of the problems with burden, it's used for boarding, taking damage, ramming, grounding and so on. This is why I find it strange that 64 gun ships with almost the same number of crew as a 74 has lower burden, which lets them enter the world of frigates.

I'm therefore very hesitant to give any frigate burden 5 unless specifically that frigate has some historical reason. Ironsides has one. Indefatigable was made from a 64 gun (which I think should have 5) and all actions she saw had numbers like 50 wounded or dead on the enemy, 3 wounded on Indy.