No, it isn't "actually more like 50%". That's just cheap hyperbole bullshit.
It's a lot more like 1% for most classes/specs. If you've got multiple ones where the difference between best Covenant and worst is "50%", post 'em, and they better be up to date, not some nonsense from three months ago in beta.
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Yeah if that's what he means, he's basically straight-up lying by using the language he is, because he's comparing a percentage about overall DPS to one about relative DPS. The difference between a 1% and a 1.5% DPS increase is "50%", but I'm a fucking liar if I go around mouthing off that the second covenant/soulbind is "50% better" without making it incredibly clear that I'm talking only relatively. Especially as many people have been claiming 20-30% DPS differences in the beta (some of which were even, long ago, true).