I run also tomcat workers behind HAProxy. I am not aware of support for AJP and even TCP for that matter. TCP may be possible though.
I forward the request via HTTP and that seems to work just fine. Yes AJP is more performant because the request is forwarded in binary, but I think you have to do something very special in order to notice the difference or benefit really from the performance gain.
If you run some standard web application you should totally get await with HTTP proxy. I think the biggest performance hit will come from running the java code and generating the page anyways, not from the protocol that is used to connect.
Also keep in mind that AJP protocol cannot be encrypted, if that is required.