I run SCM40As and love them. However, in your situation I think there are a couple of “buts”.
Firstly, how you rate the ATCs vs your active SL2 setup may depend on your tastes in music. Most of my listening is to classical, jazz, acoustic folk where the wonderful ATC midrange driver is at its most revealing. I love the natural ‘musicians in the room’ sound they produce, the tiny details they reveal even in complex musical passages and the feeling of limitless headroom they exhibit. The way they reproduce piano is nonpareil in my experience.
Secondly, I came from 250DR driving PMC Fact 3s, so considerably lower down the Naim hierarchy than you. If you’re seriously considering ATC actives (and your room is suitable) you should probably be thinking about active 50s to stay at the same level. ATC manage to maintain a clear family sound throughout their range but the bass goes lower, the amps are higher quality and you get ‘super’ versions of the drivers. They cost a lot more of course!
I agree with HH that a home demo is essential. I would try to find a good dealer who stocks both Naim and ATC and arrange for a shop demo and then try them at home. That’s what I did and it has certainly paid off.