Does the grayscale feature of iOS 8 extend the battery life?
There are two approaches that could lead to less power consumption:


a display that is enabled to have a dedicated greyscale mode; less pixels to be in charge of
software that automatically renders "less" if you tell it not to render any colors


Whilst the first is definitely not applicable as the iPhone features an LCD screen, the latter is something I once inspected. I made an iPhone draw a couple of dozen lines per second and then print how many it could render  in 30 seconds.

Having the accessibility feature enabled I hoped for a higher score in my little "benchmark". However, it resulted in no improvement at all. Actually the iPhone was slower having grayscale activated.

It might be misleading but a lower benchmark, here, indicates more CPU cycles, hence more power consumption.

So unless the iPhone actually caps CPU power on grayscale mode (which I doubt) your iPhone consumes even more power in grayscale more. This is probably due to the fact that it first renders the view in color, then caches it and puts the gray-filter on it.

Side fact: on iOS9 you will have a "low power mode" available for your iPhone. I already use iOS9 on my main device on a daily basis and the power saving functionality is awesome! My iPhone survived a 6-day music festival whilst taking pictures, checking mail in the mornings and sometimes calling people. So don't worry about it too much, in a few weeks your iPhone will last surprisingly long.