At heroic item levels, on average probably around 1-2% per ilvl.
Simcraft already updated the DPS rankings with +8 ilvls, CollisionTD posted them here:
Thread link.
Expect at least 40k upgrade at BiS, some classes gain way more than that.
Edit: As far as impact on boss strategies, a lot more guilds will be able to(on 25HC) to push Garrosh to trigger P3 before 2nd transition happens. Our best tries so far have been around 16% with no Lust.
Not only would this make the fight a lot faster(we afk 2nd transition), but also a lot less silly wipes due to the Monk tank not having to kite adds until P3 is half way over.