& looks faster than %

Yes, you're right - complexity will stay at O(N). I used wrong words trying to explain that passing the list twice is not necessary.

It is, but it won't help in this Kata since the input is limited to 12.

I'd suggest to use lambda. Why sorting a list twice?

Even if you iterate through the list twice, the time complexity is still linear

You're passing the list twice - worse time complexity :-)

Passing the list only once + reducing costs for additional list allocation...