Some people add a smaller version of their logo to only show when viewed on mobile devices. So if you’re interested in doing this, all you would have to do is add a smaller version and place it in your images folder……then link to it in your css in the responsive section.
You could also add background-size to your site-header/title class in your responsive section as well. Sometimes it works, sometimes it seems to stretch the image and distort it a bit…..but it may work for you since you only need it for when the iPhone is in portrait mode. It looks fine in Landscape.
I assume you already know how to do the above, but if you need any assistance with any of it just let me know.
Your site looks awesome by the way! Are you interested in adding this to the Showcase when I have it ready?