I don't think it would be worth it. What pieces are you specifically looking at? The only off-set pieces that are worth a darn in place of tier are the shoulders off shamans and the chest off paragons, and those are negligible at best. If you are replacing your heroic tier gloves for hwf, the mastery / haste itemization would be better for you, even if it's 6 ilvls below. If you are attempting to replace heroic tier with hwf spirit crit gear, you'll likely see a decrease in performance.
I see, however I don't think the terms would change. The only off-set pieces I imagine that are available to him are the spirit crit ones in the beginning four and then the mastery crit gloves off galakras. Until you got heroic tier gloves and dark shaman shoulders, I would imagine using the four set with tier shoulders and the gloves off galakras would be an increase. Any use of the spirit crit gear would likely be a decrease as you are giving up better itemization gear in favor for an intellect gain. At this gear level intellect slightly lags behind the value of haste and mastery.