I propose an experiment to determine if this is fact or fallicy. The method is:
1. Take picture before roaching, but after brushing.
2. Take picture of roached mane.
3. Wait 6 months - post another picture of the mane growing in.
4. Wait another 6 months - post yet another picture of the now mostly grown in mane.
5. Let's compare picture number 3 to picture number 1 and see if we can tell the difference in thickness.
If any grooming products are used in the process that should be noted.
Personally, I'm not taking part, but I sure would be interested in the results. My horses need their manes for the flies here LOL.