18% is still a very high shooting percentage, look at all the percentages I posted, and those are of the top scorers in the league. He shot 13% last year, if he had done so this year that's 27 goals. An improvement but not much of one.
I don't know how shooting percentage works with age in junior, my guess is that it goes up for two reasons A: improvement in skill and B: being used more on the power play. I don't think it usually goes up by 80%+, though.
I don't care about Filmon being a sixth round pick. Obviously he had a phenomenal D+1 season for a sixth round pick, the best I can remember in 20+ years of following Devils prospects, but that has no bearing on what he is now or what he becomes.