Cataclysm bosses aren't designed to work that way. A moderately-geared tank should not be in grave danger from Cho'gall producing 40K melee swings. The danger comes when Cho'gall uses Flaming Destruction for 80K while hitting with a melee swing. In that situation, the paladin block doesn't help against the magical damage, but the DK's tools actually can help him survive. And for either the paladin or the DK, stipulating low incoming heals is a bit unrealistic.
Heroic Cho'gall and Sinestra have hit me for 80-100k before (100k being rare), without flaming destruction.