I've been using Vim for a long time an unfortunately I've got very used to using <S-Space> to exit insert mode. Old habits die hard.
It seems when I put
imap <S-Space> <Esc>
in my .xvimrc it ignores the Shift part as interprets it as just Space. I think some time ago I read somewhere that that's an issue with OSX (they just didn't think anybody will ever need to type a 'capital space').
The way I solved that with Vim in iTerm2 is just map a character I'll never use (✠) to <S-Space> in iTerm2 and then use that in my .vimrc.
Any idea how I can make <S-Space> work in XVim?