My only concern about this is that people are deprecating Vim stuff in favor of NeoVim. My experience with NeoVim is that it's not ready to replace Vim yet - I'm optimistic about NeoVim, but it's too early to start deprecating Vim stuff.
And when did you last try Neovim? Because I use it everyday with all of my plugins from vim with 0 problems. The new remote plugin system with python support is making huge strides. People have already started on Go support and I imagine JavaScript, lua, etc is not far behind. It's a very exciting time. I recently even got a commit merged, it's a great community and I've plastered my Macbook with Neovim stickers. :P
That's not needed. With the default config or a simple vimrc it's a drop-in replacement. Some extensions don't work yet (even Syntastic) but that should be resolved soon.
However there's no difference so far: Syntastic checks are still synchronous, and I should spend some time to research alternatives (e.g. http://www.reddit.com/r/neovim/comments/2m1w3k/neomake_an_as...)
The only ongoing works on the problem I've seen are https://github.com/scrooloose/syntastic/issues/1253 and https://github.com/benekastah/neomake, which I've not tried yet.
It would have to be written using the new remote-plugin architecture I'd think. Plugins that were written for Vim would always be synchronous.
I would guess this feature is already in NeoVim.