I got bored waiting for the USA vs RUS game, so I compared Senyshyn's regular season goalscoring rate to a select few (mostly) high-pick OHL alumni that are now considered solid NHL players. Bear in mind that age is not considered here and it's far from comprehensive, seeing as total points tallies aren't considered - but it still reflects fairly well on Zach.
Senyshyn - 91 goals in 162 games (15th overall) - 0.56 GPG
Perry - 140 in 253 (28th overall) - 0.55
Kadri - 92 in 242 (7th overall) - 0.38
Schiefele - 84 in 158 (7th overall) - 0.53
Skinner - 77 in 127 (7th overall) - 0.60
Bennett - 65 in 128 (4th overall) - 0.50
Duchene - 61 in 121 (3rd overall) - 0.50
Landeskog - 60 in 114 (2nd overall) - 0.52
O'Rielly - 35 in 129 (33rd overall) - 0.27
I'll compare him to other guys that had elite speed:
.49 GPG Dan Paille - was also gritty in his own right.
.42 GPG Rico Fata - was solely a speed demon.
.70 GPG Emerson Etem - looks like elite speed carried him?
Trying to think of other "pure" burners?
Aside from Duchene most guys you have listed Skinner Perry Schiefle all had speed as a area to develop. Skinner if I remember was really quick amazing laterally and on edges but his high end speed was questioned. Perry always fought an awkward stride. Schiefle was body strength and skating.
I've watched 2 Soo games on TV and man can he flat out fly. Obviously speed always transitions and is nice to have. From what I've seen a lot of his goals are on the fly using speed and angles. Which is fine but it will be very interesting as he transitions to the pros.
Being physically dominant or have one skill (skating/shooting) can carry you a long ways in Junior hockey.
The guys that like him are gunna say I'm picking him apart but here's to hoping he's more Marchand/Duchene like transitioning.