When Should I Break Set Bonus?

In my drive to break the mystical ilevel of 500, I ran 2 different LFR raids. In each, I was lucky enough to get a new loot drops that are upgrades a current item.

Hands: Sha-Skin Gloves(483) -> LifeDrainers Sordid Grip (502)
Chest: Sha-Skin Robes(483)  -> Robe of Midnight Down (502)

502 in my bags, not on my back.

I haven't equipped either piece yet, because they'd break my 4 piece set bonus (and because AMR doesn't say to). The way I currently use Dark Soul: Instability is to buff my Chaos Bolt spells. Build up 4 full embers, trigger DS, then fire off 4 Chaos Bolt, watch for crits! (Saw one crit for 900k damage, awesome!) This increases the CB damage typically by about 30% (250k to 400k damage each time).

So, when does the overall gain of jumping from ilevel 483 to 502 outweigh the gains from the set bonus? Or am I way over due and now that I have 2 better items, I should replace them both?