Interestingly all models I've tried have a different heater "sound signature" and produce different click patterns. The FM platform is constantly evolving and they are changing more details than we imagine.
I asked my contact more info about the different heaters and the answer I got goes in the sense of what I wrote above: for instance the FM5-S and the FM5-Pro have the same nominal power output of about 20W (7.4V*2.6A=19.24W nominal, close to 22W when full) and more surprisingly, they have the same battery cells (I kinda assumed the pro had cells with a higher C discharge rate and a more powerful heater)
So why is the Pro heating faster than the S then? This is where it gets interesting: I was told that the Pro has a thinner ceramic bowl! That's an important difference to take into account.
Another interesting point is that the Mini-Pro with its peak power output of 29.4W is more powerful than the S and the Pro (and most probably than the Mini) The champion being still the FM8 with a peak power output of 120W and 40W on average during use!
My contact confirmed what was reported earlier about their use of a 1800mAh LiPo cell in the Mini-Pro vs standard 18650 Li-Ions in the other models (the FM8 having a 3S LiPo of custom shape) This is due to the space taken by the OLED screen. Hopefully the new models with only LEDs on the side won't have the issue. But it's a matter of trade-offs: we lost about 30% capacity but we gained power in the process...
What I said about the battery level meter looking linear wasn't very clear: I think it's a linear mapping as the level looks directly correlated to your average Li-Po discharge curve:
At first you get a rapid drop of the meter from full to 3/4 full or more, then the meter stays for the most part between 3/4 and 1/3 more or less (the linear part) But once you reach the last 1/3 (about 3 or 4 pixels on display) then the voltage drop is super fast, and you end up with nothing and a shut-off in no time!
If they used some kind of more elaborate mapping between the voltage and the display, that last 1/3 should only be displayed as a single pixel in the gauge, as when you reach it you are already 90% discharged. Don't know if that is any clearer heh!