Sorry Tom, muscle aren't just muscle. There are different fiber types that twitch at different rates of contraction. Form equals function by training slow the pathway used If over 10-15sec (could be off here a little) is mainly ATP, Creatine, Pyruvic acid. After 15sec to about 2-3 minutes is mostly glycolysis, after 5 min is mostly oxidative, each muscle fiber type (there are three classes) uses a distainct pathway more efficiently than the other fibers. So by training in the time range 15-60 sec is mostly a Pink fiber (IIa-?)type which promotes good size gains and oxidative ability, but not as much speed and power output as white fibers. Also there is an issue of CNS training, when you train slow the muscle fibers are "taught" not to fire all at once since the musclular contraction has to be longer . I'll put it in one simple anology: if training with a 3,0,3 cadence is helpful with peak power then why don't most top powerlifters train that way, it would be safer then heavy triples and singles, they don't because the need explosive power...
You are right in the part progressive resistance will add muscle, so can the CNS as well