Is the gain worth it on a stock motor, though that could be debated I would say no it's not worth the cost to switch for such little gain on a stock power level. E85 is far better fuel for when you want to be pushing more power and want to be able to run knock free all the time.

A simple tune and some quick maintence stuff like colder plugs, better wires and a intake could gain you more than just switching to e85.