I'm not sure the idea of a "User Interface Designer" instead of a web designer will not fix a startup. Folks like Steve Blank and Eric Reis have been saying for a while that (for a web app) user experience is where a lot of startups fail. Eric Reis talked about having customers come in to their office and sit next to their engineers and talk through user experience. Steve Blank talks about "getting out of the office" and talking to your customers.

The answer to better user interfaces isn't having another "expert" in the office with a fancy title. It is making everyone in your company into a "user interface expert" by bringing in the real expert, the user.