Late reply but I'd like to elaborate on this a little.
40 ilvls is a huge jump and probably would be enough for any class to make the switch, no matter the slot, no matter the secondaries.
Anyway, frost has it pretty good, in that even our worst secondaries are pretty decent. Multistrike is definitely strongest by a mile (has a stat weight of .62 for me, assuming Int is 1), but even our worst secondary has a weight of .40-.45. My hunter friend simmed his character and his worst secondary had a weight of .22, which is just awful.
Because of this, you usually go with the piece of gear with 10+ ilvl, despite the stats. The mythic crit/versatlity belt off of Brackenspore was a ~120 dps increase over the heroic mastery/multistrike belt from trash. Pretty small, but still an increase.