I also use a Komplete Kontrol S61 MK2 with FSP8. Everything works well in this combination. And since I also use Maschine Studio and the MK2 keyboard works perfectly with it, there's no MK3 for me right now. Unless FSP8 supports it as deeply as Cubase.
My further experience with Komplete Kontrol V3 vs. V2 as a plugin and its compatibility with the MK2 keyboard is as follows:
In KK 2.9.6, everything runs like clockwork. My workflow with FSP8 involves having, for example, eight instances of KK 2.9.6 open, and each instance remembers its location from which it loaded samples, instrument presets, effect presets, etc. This allows me to quickly select an instance and use the Preset Up/Down buttons on the MK2 to cycle through the sounds, samples, or presets, as I'm always at the correct source location, i.e., within the NKS ecosystem.
I can't get any of this to work with Komplete Kontrol V3. The locations are forgotten, and as soon as I press a preset button, it jumps to the very first sound. It doesn't remember anything.
Furthermore, I've noticed that I can browse through samples incredibly quickly with the MK2 keyboard and Komplete Kontrol 2.9.6, using the dial. With KK V3, there's always a slight delay; I can't browse the samples at the same speed.
Long story short: I'm waiting until FSP8 achieves full compatibility with the MK3 keyboard, and then I'll try KK V3 again. But I'll probably keep the MK2 anyway, also because of its perfect Maschine Studio integration.