funny you mention this as I have the Mato sherman and I upgraded the idler and drive sprockets with metal versions. The tank did'nt run well on the sprockets at all and kept jumping off. I think their is a slight mis-match between the metal and plastic drive sprocket teeth that causes the track to jump. Another thing I noticed is that the idler wheel in metal is much thinner than the plastic one, causing the track to run irregularly. Basically the metal drive and idler wheel upgrade was a waste of time and money, since the tank ran fine on the plastic versions - I've switched back to them. This is not the first time I have had trouble like this on metal upgrades and would advise caution when doing it.
Having said all this, I do have a tiger 1 and panzer 3 that run just fine on metal tracks and sprockets etc.- weird.

CaptB