Yeah I agree with you on this. I reached out to Abyss and they said that the 1266 scales with head room but I should be able to get sufficient volume out of it since I was able to use the HE6-SE with the FA-12 without issue. I think, like you suggested, that I will wait to see how the 1266 sounds out of the FA-12 before deciding if I want a more powerful amp or not. I have other amps that do up to 6w but they don't seem to have the current that planars like. I used to use the HE6-SE with the THX 789 which does 6w at 32 ohms but using it on the FA-12 feels like this amp is driving it better I'm guessing due to the class A design.I don't own FA-12 but I know it is around 2.5W. I doubt you will get the most from Abyss 1266 from 2.5 Amp in terms of Bass depth/dynamics etc. as it requires a lot of headroom. Since you already have FA-12, would recommend to pair with Abyss 1266 first to see if it works well and if does not then you can buy another powerful amp.
Also I asked Vitaliy what he thought about the FA-10 or FA-10 pro. He said that he prefers pairing the FA-10 pro with an r2r dac as the FA-10 with an r2r dac could be too warm but obviously this is a very subjective decision. However if I end up with an FA-10 for use with the 1266 it would probably be the pro model. Also he said lead time for the FA-10 pro to ship is 1 week.