The 4pc bonus isn't very strong to begin with, I wouldn't be concerned with dropping it.
I have been passing on upgrades such as Hauberk of the Towering Monstrosity because taking that piece would cause me to lose the 4 piece bonus.
My question is, when does an upgrade become big enough to permit losing the 4 piece bonus?
edit - wrong forum =(
In comparison with the other available pieces? Nothing.
There is very little reason to reduce the CD unless you are a frost tank that uses the ability every time it's up. Otherwise, with the way encounters and boss ability timers work, you gain nothing.
Generally speaking it depends on your spec, you are blood so the 4 set is worth:
Without bonus: 10/60 = 0.166667
0.15*0.166667 = 0.025
So if used on cooldown (which is doubtful) Vampiric Blood is worth 2.5% extra health.
With bonus 10/50 = 0.2
0.15*0.2 = 0.03
So if used on cooldown with the bonus VB is worth 3% extra health.
0.5% extra health for the 4 set, keeping in mind this is IF you use it on cooldown. When tanking I rarely find myself doing that, depending on the fight I save it for when healers are under the most pressure (ie Jaraxxus somebody gets debuff, and there are infernals stomping around the raid) There are times when the 4 set is indeed handy. If Patchwerk was a boss in ToC, then it would be great.
NR Beasts: Non need for it, the cooldown is up whenever you need it for tanking on Gormok, and you can alternate AMS and VB for Acid Spits.
Jaraxxus: Generally saved for important times, so cannot be used on cooldown.
Faction Champs: More health is bad for tanking these, don't use it unless you are in real danger. Their AI is more likely to go for low HP and low Armor targets. I swap to BP for this fight.
Twins: Generally saved for times when needed, don't see the lower cooldown really helping if it goes unused for 30+ seconds.
Anub: You might be able to squeeze out a 2nd VB once per burrow phase, but upon reaching Phase 3, VB should not be pressed. Just to avoid unwanted Leeching Swarm healing.
All in all, not the greatest bonus, though it does have a place. If you can get 245 Beasts chest and Jaraxxus leggings, drop the bonus.
ICC 7/9 http://www.hysteriaguild.net/ -Recruiting CasterDPS(esp SPreist)