It's actually a three-way debate as to who was the best offensive defenseman of that generation between Cameron, Cleghorn and Boucher (sort of surprised you forgot him but the numbers bear this out). Cleghorn has the most impressive NHA stats of the bunch, but the worst NHL stats, and I tend not to count NHA stats for quite as much. It's actually pretty hard to distinguish one from the other offensively.
There's no reason to assume that the NHL stats from that era are any better than the NHA stats.
Does Boucher really have the offensive resume to join in this debate for best
offensive defenseman of the period?
In his 7 NHL seasons as a defenseman pre-merger, Boucher's rankings in scoring among blueliners are, stated in order of impressiveness, 1, 1, 2, 3, 3, 4, 5. These are of course pre-merger so they could be essentially doubled if trying to compare to a post-merger league. Post-merger, an aging Boucher finished 9th, 11th, and 11th.
Cleghorn was almost never out of the top-4; he actually did it for 13 times in 16 seasons: 1, 1, 1, 2, 2, 2, 2, 3, 3, 4, 4, 4, 4, 5. These are pre-merger seasons too, so they are apples to apples with Boucher.
percentage-based comparisons would help to indicate which of the 1sts was most impressive, as well as sort out the 2nd place finishes better as well - but
is that really necessary?
It's like comparing Gary Suter's offense to Lidstrom's.