Good to hear you have positive experiences with Firefly. Regrettably mine are not so positive as I had 8 of them in our Amel 54 but they were not up for the task. The voltage was getting below 24 V when the SOC came below 60-65% and we had any kind of load on the system (fridges mainly). In my case the internal resistance of the batteries seemed fairly high as the voltage dropped significant even at loads in the region of 10-25A.
The guys from Firefly sent me a diagram that made it clear to me that these batteries are typically 0.2 V below AGM batteries and any SOC. After some discussion with them they basically told me these batteries were not suitable for my use-case.
So for me this meant an usable capacity of about 120-150 Ah, not great on a power-hungry Amel 54.
So after a year I decided to replace the batteries with 24V lithium units. Safe to say this was a very expensive experience.