You may be interested in the experience I and two friends had on the gravel/dirt sections of the Cassiar a couple of summers ago. Three types of bikes, three results. Note that it rained much of the time and the road surface was somewhat slimey.
I rode a Kawasaki ZR-7S which, by virtue of geometry, tire size etc, would be roughly equivalent to a sport tourer. This bike was not at all happy. Above 40 kph it felt squirrely and insecure. I'd attribute this largely to the fattish, sportsbike like, 17 inch tires. I could have, should have lowered the pressure in the tires but could not be bothered. However, as long as I kept the speed down it was ultimately O.K.
One friend rode a BWM 650GS which ordinarily could blast along gravel roads with no issues. However, the bike had only a high dirt bike style front fender which was hopeless at deflecting the silty spray. This rider was down to a miserable crawl with many stops to clean off his face shield. A complaint to BMW after the trip resulted in a free additional low fender.
The third bike was an early eighties Honda CX650e which did surprisingly well. My assumption is that this was due its larger (18 inch) and narrower (old style) tires and the fact that it is a V twin which, I understand, provides a more gravel friendly power pulse than the spinny four cylinder on my Kawasaki.
I'd second the recommendation of a side trip into Stewart and Hyder. These twin towns on either side of the border have become something of a destination for motorcyclists and you're bound to meet some fellow travellers.