Holy Knights should be probably limited with high level training similar to Fishman Island Luffy, thats as far as someone could get stronger without the real battle experience.
If Oda doesn't contradict, this is what Rayleigh says:
If Holy Knights don't get experience of extreme conditions of real battle, they can't be top tiers with training alone.
Its some chance they might be fighting vs Revolutionaries or some other rebels in real battles, if thats the case their level might be higher.
Its speculated Gorosei should be strong since they have battle scars meaning that they have battle experience, if Holy Knights are like that then they could be top tiers.