Some gain more than others. Some, for whatever reason, do not gain at all. Since this subject is still a work in progress, it's difficult to say whether everyone can make such gains, or whether there's some sort of bell curve to potential gains.An ex member of Thunder's place grew from below 6" to over 11" he is called DoubleLongDaddy - he has now started his own PE website.
LPSG member Motlissof grew from around 6" to over 10" so I'm not convinced that there are limits to growth.
All this happens over several years with consistent PE sessions!
(I'm certainly not discouraging anyone from trying!)