How can I use Homebrew to install MongoDB 2.8 with "Wired Tiger" storage engine on my MacBook?
You might have to wait for whatever volunteer updates Homebrew recipes to adopt 2.8 or perhaps pitch in and submit an update to the repository to bump the development version to 2.8.

There aren't any other competing packaging services that are as active as homebrew so watching there might be your best bet if you don't care to DIY.

If you have brew show the recipe for mongodb you see this issue as to why --devel is pinned at 2.7.7: brew cat mongodb


# This can't be bumped past 2.7.7 until we decide what to do with
# https://github.com/Homebrew/homebrew/pull/33652



You could also weigh in there to see if there's something you could do to help unwedge this recipe. My guess is you'll need to just download the specific unstable 2.8 build you want from mongodb directly:

https://www.mongodb.org/downloads