I need help explaining to my boss why her design is poor on a client\'s website. She has no knowledge of the web, and it can be difficult as a web developer working with a
Hmm. It sounds like you guys need a requirements analyst to step in the middle here. Deciding on a broswer specification & resolution that you'll conform to is a fair thing to ask, I think. Just assuming that 'most' users will have wide screen is not enough for most apps. Seems like she'd be hard pressed to explain why she can't redesign her banner to be smaller & fit the desired size.
I think that user will absolutely notice the horizontal scroll bar and be annoyed by it. Because it's not something most users are used to seeing (can't think of any major sites that have one), they'd have (in effect) learn something new to use your site, which is not good. They should be able to look at a site and be able to use it right away, not spend a few seconds figuring out that the scroll bar doesn't show you any new content, just the additional graphics from the header - those few seconds are where you lose people.
I wonder also, if there's any section 508 guidance on horizontal scroll bars. That may not matter to you guys, but I'm developing gov't sites, so 508 is a big deal for us day to day. If you've got a user using just a keyboard or a screen reader, that scroll bar is more than just annoying.