Argument for/against splitting login process into checking username, then ask password?

My boss has insisted for a while that our login process be divided into two steps on separate pages: on the first page, ask the user's e-mail address. On the second page, either display "user found" and ask for password, or "user not found" and ask for e-mail again.

He insists it is much easier for our users who often forget their credentials or if they even have an account yet. I've long been against this approach because it forces every user to go through two steps every time they login. (I also wonder if it's a security concern, knowing that it will confirm when an email exists in our system.) I would prefer to have a more standard login page with e-mail (all of our usernames are e-mails) and password, and one very clear link for "forgot password?".

Neither of us have any hard data to support our theories, and we have more important things to do than A/B test something that he doesn't think is even a problem. I was just wondering if anyone here could provide some arguments or hard data for or against either approach. I enjoy considering UX design, but I am not an expert.