To provide some perspective, Highway 7 at the boundary between Guelph and Waterloo, which is slated to be replaced by a 4 lane expressway, reaches aadt volumes of 22600. Is on par with Highway 11 between Orillia and Gravenhurst, which is presently an expressway, and far busier than the section to North Bay which is currently being upgraded. It is far busier than the any other section of Highway 17 save Highway 417, even those parts 4-laned around Sault Ste. Marie and Sudbury, as well as Highway 69 between Parry Sound and Sudbury (also being upgraded). It's busier than Highway 400 north of Barrie, most of Highway 402, all of 405, 416 south of Ottawa and 417 east of Ottawa.
Granted, many of those are rural highways (except most notably, the 405); however, I do think that some sort of improvement is warranted around Thunder Bay. I suspect the biggest reason those parts don't receive more improvements is that there are only two ridings in the area. Highway 400/69/11 improvements buy votes from everyone in the GTA who has a cottage up north.
FYI, my assessment was based on a quick scan of
this report from 2005.