I used to carry 4-8 mm PrimeAgra, which I thought was better for small plants and seedlings than the standard 10-16 mm material, but found out that it wasn't necessary at all.

AV's LOVE S/H!

I'm not sure what you are specifically referring to about a "poor root system", but keep in mind that when you move a plant from one medium to another, it's going to need an "adjustment period" during which it grows new roots that are optimized for that new environment, while the old roots - not optimal in that same environment - will struggle to support the plant and will ultimately fail. A plant with "iffy" roots will have less time to spend growing those new roots, so you have to take extra measures to keep it going, such as putting a plastic bag over the plant and pot to contain the humidity, so it won't desiccate itself to death through normal transpiration. I think it also pays to treat the plant with a good root-growth stimulant like KelpMax.

Having said that, if the plant is being moved from a poor environment like soppy sphagnum, into S/H, the plant will take to it like mad.