DCP,
I have a newer GCIV with single shimmed pockets, I have a friend with an un-shimmed GCIV, and a friend with double-shimmed GCIV. I and the 3 professionals that I know prefer the single-shimmed table for an amateur player. The un-shimmed pockets are WAY TOO big. The double shimmed pockets are much smaller even than the tight Diamond tables, and it is VERY difficult to practice any difficult position shots on this table - the guy who owns it also has a double-shimmed 10 foot table, and after 15 years he can't play position worth a crap. If you want to improve your pocketing skills, then the tight pockets are great, but why not just use those rubber pocket inserts that you were posting about when you want to practice your accuracy?
The single-shimmed GCIV has pockets just slightly smaller than the tight Diamond tables, but despite this plays easier than the Diamond table (the Diamond pockets are cut way deeper, and the angle of the pockets is slightly different).
When they shimmed my pockets, they took the rails to the shop - said that it was better to do it there than on-site at my house. This might be a problem for you. I know the name of a super mechanic (sets up table's for Jay Oerner & Sons, as well as several pool halls and several professionals) - P.M. me if you need to get in touch with him.