I would go with metal gear servos. As far as I know, there is no metal gear upgrade for the servos you list. I would shelf those karbonite servos you have (or sell them on ebay) and pick up hitec 625/645MG or digital 5625/5645MG's. They are going to be the most economical route as far as quality metal gear servos, and as long as your aren't doing alot of 3D the 645mg's would work fine and are really inexpensive. If cost doesn't matter as much, then get the 5925/5945's. I would put a hitec 5955TG on the rudder. Yes, its big bucks, but then again so is the rest of your plane and engine.