163 Protest has more sidecut than the other lengths, at 25m radius. Which is exactly the same turn radius as the 169 RX. It's also 119 underfoot, essentially a -10 regular protest. So your girl could...

So many people going -10 RX. I can't wait to hear how they ski!

By like 5mm Completely unnoticeable, IME, speaking as someone with a 303-307 bsl who has swapped loads with people in the 315 range.