I'd be interested to know what the bell curve off MLB average spin rate looks like.
Those two numbers don't really mean anything to 99.9% of people that follow baseball
In the sample of 315 pitchers Baseball Savant lists as starters, Arrieta ranked 73rd in average spin rate and Slegers 290th. But spin is kinda like velocity in that just because you have a high number doesn't mean you're going to be a good pitcher. Nik Turley had the third-best spin rate in that sample and Dallas Keuchel was 270th.
Again I'm not a spin expert, but from what I understand there is such thing as effective spin. There may be a certain gap between your pitches you want to shoot for in terms of spin rate, so more spin isn't always a good thing. So to really break a guy down in terms of effective spin it seems you'd want to look at each of his individual pitches, which is a little out of my league at this point. But I think it's safe to say Slegers would likely benefit from more spin.
The good thing is we know guys can effectively mess around with these things to a point. The Twins most certainly learned some things from having Craig Breslow around last year. Maybe they can take some of those lessons learned and apply them to a guy like Slegers and see what happens.