Just a quick update: Blizzard released a new build of Patch 3.2.2 to the Public Test Realms today, and MMO Champion kindly informed us that it contains another tweak to The Beast Within:

The Beast Within now reduces the mana costs of all spells by 50% for 10sec. (Up from 20%)

This change is not unexpected — Ghostcrawler mentioned the possibility when discussing the last set of changes to The Beast Within. And now it looks quasi-official. (Although note that nothing on the PTR is ever guaranteed.)