You could already single tank Magmaw by cycling between Divine Shield -> Guardian -> Hand of Protection -> Guardian -> Divine Shield -> etc. The cooldowns worked out exactly right even with a 2 min forebearance.
---------- Post added 2011-02-09 at 11:14 PM ----------
4-5% of total healing done is actually a very large amount. Individual healers have to upgrade a lot of gear to get that kind of performance increase.