Thanks for some excellent feedback so far. Boss mechanics inevitably involve some breakpoints (alas, there's no way for Malkorok to create precisely 3.7 Implosions), and they do need to scale in some form, or abilities only targeting maybe 2 players out of a 25-player Flex raid would make some of those mechanics feel completely trivial and detract from the intended experience and tuning. In general, we try to err on the side of rounding down, and making sure that the ratios are never worse for any group size than they would be for a normal 10- or 25-player raid. Norushen orbs are a great example of a place where that logic doesn't quite work, though, since you actually want more orbs, and not fewer. That's something we can adjust.
Personally we have found 11 to be just about the hardest number. The difference in scaling between 10 to 11 is about 11mill HP. Now for a boss like the Iron Juggernaut, that’s fine. You pew pew the boss the whole fight. 11 mill over 5-6 mins is do-able. But, for a fight like Galakras where the boss is a burn at the end. That 11 mill is pretty tuff to make. Especially being flex so the extra members are not exceptional.
TL:DR: the boss HP Scaling is a bit over tuned for 1-2 extra players. (on some fights)
A bit of clarification here: Boss health and damage scaling are both completely linear and proportional to group size, but scale up more slowly than raid size does. So, all else being equal, if you add players who are roughly on par with the average gear/skill of your current group, you'll have a slightly easier time as a result.
Thok, for example, has 355 million health with a 10-player Flexible raid. If you add an 11th player, his health will go up to 390 million. I.e., you've increased your raid's damage output by somewhere between 14 and 20% depending on how many healers you were running, but the boss's health only went up by 10%. Adding that 11th player will also make Thok melee for about 3% more damage, which will barely be noticeable.