Haste will continue to give a linearly increasing benefit past 1GCD because of it's effects on DOTs and cast times, since those never cap or diminish. You cannot have too much haste unless you are ignoring too many other important stats to get it.
VT and DP are huge % of spriest's dmg, there is no achievable amount of haste that will cap where these will benefit from haste. If you don't belive me, go do General Vezex and pop hero.