After my doctor prescribed a sit/stand desk, my work provided me a Varidesk accessory. I sit for one hour and stand for one hour according, following doctor's orders.
My focus is about the same from what I can tell, and I actually prefer to stand.
While standing I shift my weight and move my legs which, according to my doctor, is the point of sit/stand desks. They promote/accommodate physical activivty while working at a desk.
That said, my focus while in the office is very low overall because I work in an open office. So, whether sitting or standing, I have to deal with visual distractions and filter conversations with headphones and generated noise.
So like the OP, my body and health benefit enormously from sit/stand and if my concentration suffers, it is not to such a degree that it is noticeable due to other environmental factors.