The most slap-shot goals by percentage of goals would have to be MacInnis but total goals scored, I'd guess Brett Hull (Bobby was before my time). Lafleur had a great slap shot, Rick Vaive as well.,,,
I suspect you may well be right about that. MacInnis had a heck of a slapshot & points to prove it, Brett Hull using that shot on a fairly large % of goals scored.... other honourable mentions would be Al Iafrate, Lemaire who although a defensive forward had quite the weapon with his slapshot when he did get the opportunity to let fly (had practised endlessly as a kid using a steel puck).
Popular Mechanics back in the 60's clocked a number of NHL'rs shots, Bobby Hull's supposedly & really quite unbelievably reading 118mph & change, and that with a wooden stick. They also "claimed" Beliveau's wrist shot hit 105mph. Obviously faulty equipment, primitive, as the average speeds as measured with reliable equipment since the 80's at least are around 100mph, Iafrate, Chara, MacInnis and others hitting 105 & change, composites or woody's. Some guy in the KHL at last years All Star Game purportedly hit 110mph & change, but Id seriously doubt that, likely rigged, publicity stunt.
The big difference between wood & composite with a slapshot is that the shot is not necessarily faster, but its consistently "heavier", and very painful in stopping if it hits you in the wrong spot. The rise of composites precipitated the increased & much derided size of goalie equipment, pads, blockers & catchers, chest protectors & so on, outsized. Few players using wood back in the day had heavy shots, though there were enough, guys like Dennis Hull who's shot was fast, heavy & wild, whereas Orr's like Gretzky's was neither all that fast, not very heavy, just extremely accurate, and usually fired through a screen or deliberately targeting a blind spot most goalies had with moulded masks. Glove or stick side, about a foot off the ice, and close to the outside of the pads. Sort of whispering by if you will. Composites & the cycle changing the way goaltendings played, all BF, "blocking" not "saving" per se'. But I think Im digressing here.....