We hiked Seymour with our lab in 2004.
We hiked Seymour last year with our 3 year old Lab. Max is 90lbs and loves to hike with us but had some difficulty a few times near the top of Seymour navigating through the blowdown (there's really only a few sections of it) and finding a way around some steeper spots.
Prior to this hike he never had any issues finding his own way up a tricky section, but he had to be lifted up and down a few times during this trip and that wasn't a whole lot of fun for us. For whatever reason, his webbed paws were pretty sore due to this terrain as well. Some of the peaks he had climbed with us before with no problems were Tabletop, Phelps, Street, Nye...
It's tough to judge our dog's abilities at times; he's a giant puppy and has tons of energy, but even if he didn't, he's follow us to the end of the Earth!