Bootstrap has a class "lead" which could be used here.

Otherwise, this could be replaced with a single class on the paragraph. The styles are linked to the page-header styles, so a class name like "page-header-sub-heading" (or even "sub-heading") would work here and be less specific.

There are some bootstrap variables for font sizes too, fontSizeLarge, fontSizeSmall and fontSizeMini - are any of those appropriate instead of defining a new font size?

The font-weight reset isn't necessary.