Probably easier to get heroic tier pieces than to get a whole set of warforged gear.
You just needs 3 warforged off pieces, but I see your point. It'll be easier for me to get warforged off pieces since I run in a 25 man, but the main thing is after testing out the 4 piece on the PTR I'd honestly prefer not to have to deal with it since the duration of the buff is so short and is susceptible to input lag, so it can be pretty difficult to keep the buff up for the full duration when the boss is above 35% and you don't have shadow blades up. Not to mention the 4 piece is all but useless on an add fight like Spoils and pretty much forces you to not attempt cleaving for the 30 seconds while Vendetta's up. It probably is easier to keep it up at higher haste levels, though.
I think it's likely I'll just take the 2 piece and pass on tier to other vanq users who'd get more out of their 4 piece during progression, and just take tier to get a 4 piece to play around with when other people have gotten theirs.