Do aTweaks' Sirines have faster-casting spells? Or was it their arrows that killed your FMT? I was always able to retreat and hide in shadows before they could finish casting their charm spells.
I've only tested mine against SCS Sirines, and I had some traps to speed things up. Only some of the damage came from stealth bow kiting. But it should have been possible even without traps: Sirines have horrible walking speeds, which means you can always keep running if you fail to hide in shadows.
In my install Sirines have natural 10 walking speed, or more I think. They're wicked fast. Maybe it's because of this. For most archers you can just kite them normally (I prefer backstabbing+Blindness, it's safer), unless they quaff an Oil of Speed (which means instant Blindness out of stealth).
They use an innate ability (a cloud that blinds you) besides chasing you with Improved Invisibility. They probably use MoveToObject() to get to you. The thing is that one of them got an arrow off (each time I attacked they all came after me, and this one was invisible), I ran away but had to break stealth to drink an Elixir of Health. When I did that they got two crits in and I got poisoned while being left with around 10HP. I was using Mirror Image at the start but it ran out before they managed to hit me for the first time. They never tried Charm because I was playing an Elf.
tl;dr it works but not against aTweaks' Sirines. It's GREAT against Hobgoblins, because they can't do *anything* about it.