1000 (1320 for me, I'm an alchemist) agi will result in more healing from healing rain than the elixirs, easily.
Keep in mind, when offhealing like this, you HAVE to let MSW stack to 5, so you're wasting some MSW stacks anyway, extra haste just means more wasted stacks, unless it hits a breakpoint for an extra healing rain tick, and the ONLY reason we would care about the breakpoint is because we are NOT sustaining healing rain 100% of the time.