my educated guess will be around 5% to 8%.
if you check out the T15 4set bonus, dps increase is at least 5% from having T15 4 set.
i have to guess that blizz wants to keep all the tier 4 set bonus around the same dps gain so it would have to be around 5-8%.
Speaking of t14, How does the Raid Finder version compare vs. say normal mode illvl gear? Our guild is having some major issues getting beyond Elegon, infact we cant even down him. Was wondering if for now I'd be better off forgoing the 4pc t14 in favor of the ilvl 489/496 drops an gear or just burn all valor into upgrades? Again this is the 483 version vs. normal 489.