I finally got around to buying some #14 O-Rings to see if they would really increase compression and the results were much better than I was expecting. To test it I put my finger on the air nozzle of the Cylinder Head then then pushed doen on the Piston/Piston Head. With the stock O-Ring it would slowly go down and you could hear a little air excaping but with the new O-Ring I can't get it to budge. If I push as hard as I can the Cylinder Head ends popping off.
Overall I would say everyone with a clone gun or anyone with compressions issues should give some $14 O-Rings a try.
I got a 10 pack at Home Depot for under $3 so you really have nothing to loose.