They certainly aren't as fast as the Coyote, LS or G3 cars. I group them into modern muscle mostly because they were the start of the new HP wars and they had most of the creature comforts that I associate with modern muscle.
It could be argued that until they got faster than the BB cars of the late 60's or very early 70's, they could be argued to not be "muscle". But I still group them in even if in '87 a cheap 340 Duster could be easily as fast as a new GT off the showroom floor. But the GT was pretty fast and had AC.
Funny how time changes things. I had a Volare in '88 with a 318 that I pulled the AC off of when I did a 4bbl swap because it was extra weight. Now, I would keep the AC if at all possible.