Tier 8 set bonuses compared to Tier 7

2/5 T8 has a 25% proc rate and a 45s internal cooldown. A 3% DPS boost for most specs.
This is an outstanding set bonus, about a 3% increase for most specs. Get 2T8, regardless of what costs, ditch 4T7 without thinking twice. Also, it takes a lot to make you drop 2T8.

4/5 T8 has a 20% proc rate, has no cooldown and can chain proc. It's a 2% boost for Fire/Frostfire, 1% for Arcane, 0.2% for Frost.
If it procs, it simply makes you not lose the MBAM/HS/BF buff when casting AM/PY/FB, you don't get a new buff, it doesn't get renewed or reset. For one, that means you don't get a new MBAM/HS/BF proc message, so you need a new or updated alert mod. Also, chain procs don't change the buff timer. If you cast Pyroblast with 0.5s on your Hot Streak buff remaining and then get a 4T8 proc, it will be wasted because your buff runs out before you can recast Pyroblast due to GCD lock.

2/5 T7 is a 2% boost for Arcane, 1% for all other specs. 4/5 T7 is a 1% boost for all specs.
So, you should generally upgrade your gear, aiming for 4T8. For Arcane, there are cases where 2T7+2T8 beats 4T8 and other combinations, usually with T7 Cowl and Pants.


Source : Maxdps