I made a table.
On top, it's when it snow, On bottom it's when not. On the middle, it's the default skis.
When range is short (Rossengel Xenon 3 for exemple has a range of 5 degrees), the efficiency is high (5%).
Sometimes it's a bet.
My advice is to get in the first range (when you choose your ski, if you a high ranked you have a box woth 1st, 2nd and 3rd range). The temperature doesn't have to change too much, and you are sure to have at least the first part of you race with efficient skis.