Yeah, like you, I am planning to try this so have been stalking hempy growers. FWIW, I have read that you can get nute lock-up with the smaller perlite. "Supposedly" it wicks too much water, essentially drowning the plants as the perlite compacts. I have no first-hand experience. I read a number of forums, if I am able to relocate the info, I will post. It also indicated that the smaller perlite doesn't allow enough oxygen to get in.
As a gardener, to me, this makes sense. I liken it to growing in sand. The larger perlite sounds better as the larger surface area will allow for more oxygen penetration. My sense is that it would give the roots a firmer foothold as well.
Perhaps some experienced hempy growers can confirm/deny this.