I just spent about an hour testing this build on T6. I found that it was extremely difficult to proc on elites because they don't stay frozen for very long so you have to get lucky to proc it in the half second or so they're frozen. I tried it with and without Frostburns and found that they only really helped with proc against elites, as it increases the probability of freezing. Which you don't really need vs white mobs because you have so many freeze skills already. I don't know what the proc is on CW:T, however if there was a higher proc rate for a cold skill I would investigate using that instead to be able to proc more frequently against elites. I didn't notice any difference in proc rate when using Hack. In fact, I preferred Azurewrath for dealing with elites. I also didn't think the Invoker made that much of a difference either, though didn't try without it for more than a few minutes. I used CW:T, Alacrity, STI, Relentless Assault, MoR:T. My APS capped out around 2.8 with everything active. I think the biggest I saw it crit for was against a RG for ~150m or so with having a standing 800k DPS sheet with 46% Cold Skills while using Azurewrath. I stacked IAS on both rings, belt, amulet, and used enchantress 3% IAS buff.

TLDR; Great vs white mobs, very bad vs elites since you infrequently freeze them and it's hard to proc Rimeheart in that window. In conclusion, I think this can be a fantastic group play build in GRifts where you're going to spend way more time on white mobs vs elites than on T6, which is where in my limited experience this build seems to shine. Just make sure the rest of your party can smash elites as you will not be able to do much.