My suggestion for set bonus changes would be something like this:
2 Piece: When you frostbolt into a frozen target, you get a 100% frostfire bolt proc.
4 Piece: Icy veins now becomes undispellable.
FFB damage honestly doesn't feel that bad, and my frostfire bolts do almost 25k+. Being able to store 2 ffb procs is a lot of dmg without Ice Nova already that is on demand, instant damage.
Second bonus speaks for itself, I like to keep my icy veins for more than a few seconds, it is a long cd after all