I put a 4-5" layer of coco croutons with one of my transplants to see if it made a difference. After a couple of weeks vegging, that plant is considerably larger than my other girls. But that could be attributed to the container it's in. 10 gallon tub instead of the normal 5 gallon bucket. It's only one plant so you can't really base anything on the results but what JK says makes sense.
I'm definitely staying with the bigger tubs, regardless. My 4 girls in flower got root bound after 10 weeks of vegging in 5 gallon buckets. I would have had to water them a few times a day if I didn't transplant the 5 foot tall girls :)
I'll try the hydroton along with the croutons next time in the same size tubs, but with more plants.