Ok, I am going to show my lack of intelligence here.
What exactly is the sub-rail? And is there a diagram out there somewhere that shows everything you need to know about rails?
DCP
http://forums.azbilliards.com/showthread.php?t=210290&highlight=tedantle
The link above shows my GC III/IV work done back in 2010 or so. Glenn re-finished my rails and replaced the rubber with Artemis Intercontinental. He had me buy it before he arrived. The sub-rails are shown at various stages of extension and finish.
Glen is a real pool player that loves the game and the equipment. IMO he would be an expert in any vocation he chose. Yes he works for/with Diamond. If he wasn't as talented as he is, why would they continue the relationship? And why would RKC stay if they weren't the best? Another of his talents is historic table restoration.
He is an excellent pool player and that is why he is an expert table mechanic. As he wrote earlier, (i.e. I will paraphrase) a person needs to know the answers to the main challenges before he hires someone to do the work. That is the basis of any job interview in any industry. - How can you know if the guy is an expert or poser?
In my experience, anyone in any field that is an expert appears to be arrogant.... often. Doctors, lawyers to mention two. RKC fits that mold.
After he worked on my table we played on it for a while. He had a chance to show me a double jump shot. Jumping a ball into a ball making the object ball jump as well. I know enough to realize that is one of the MANY little things he knows. What was funny is that the stick he was playing with had a tip/Ferrel that was loose. He's an old road player know doubt. Straight pool was his favorite starting out.
What impressed me about Glenn is the passion he has for what he does. He loves to share his knowledge. He is a perfectionist above all. The work he does is hard physical labor. I know, because I helped him. I was there every step of the way. A couple of guys came in from Ohio just to observe his technique and equipment that he uses. Specialized equipment that he built himself, to consistently plain sub rail systems accurately. He calls it Diamondizing Gold Crowns. His friends at Diamond were not excited about that part of his work.
I have also worked with Jerimy Chambers here in Chicago. He is hands down, the best in the area.
On another note, I have played with a top rated world player that said he didn't like the way Diamonds bank.
If Glenn says "Mark" is capable of accurately adjusting your GC rail systems, and you like Diamonds.... I'd go for it.
So make a decision. If you like the way Diamonds play, buy one or have your sub rials extended and re plained. If not shim the rails you have. Done.