You haven't tried some of the heavier auto-complete plugins. As it stands right now, they have an annoying tendency to block the UI while you're typing! It's bothered me so much that I've even considered writing my own vi clone based entirely on an asynchronous message queue such as this!
The redraw issue is also related to vim not being thread-safe.
With this fork youcompleteme can probably do something like 'vim.defer("YcmTriggerSyntastic")' from a background thread to invoke syntastic and redraw the screen.
