I think you already know the answer to your question, but lets see if I can reaffirm it. Start interviewing, there's tons of backend work available. At a new company you'll have the specific role you're looking for, and you most likely won't be asked to take on out-of-role projects as the primary developer.

As far as OS level stuff goes, I can't really answer that question. If you think you're good enough, figure out a way to impress people with your compiler work. If your work on that stuff has some potential, I can't see why a company won't give at least a phone screen.

Just take it easy, send out a few resumes every week, see what kind of interest you get. Good luck.