As a raiding warlock with both t4 and t5 4sets i can tell you all that t5 4set is much better as for dps output.  In most boss fight i am in the top 3 on the dmg meters whereas b4 i had the 4set t5 bonus i was in the top 6.  the set boosts my corruption ticks for an average of 850dmg.  I have seen my corruption tick for over 1000dmg in certain circumstances. im an afflock, so i havent really had the chance to test out the improved immolate. theres a reason why  4set t5 is harder to get than t4