AEG spring guides are often over looked on an airsofter's upgrade path simply because they do not directly cause any increase in performance. Instead a spring guide is designed to prevent a variety of malfunctions and to release stress from the gearbox as it cycles. Upgrading your spring guide will do wonders for an airsoft gun's durability and life span, but not for it's performance. Spring guides are proprietary regarding what version the gearbox is.
Metal Vs Plastic Spring Guides
Many stock spring guides are made from plastic and have a single washer that is fitted on them to keep the spring from cutting through the spring as the gun fires. While a plastic one will keep the spring in proper alignment with the piston, they will eventually wear down and break. A metal spring guide will never have the issue of wearing down and will always keep the gearbox internals in perfect alignment. Upgrading a spring guide is largely a preventative measure, a cheap or broken spring guide can cause lock ups, full jams, strip gears, snap the spring, strip the piston or cause a variety of other unusual problems.
Bearings are a very important part on a spring guide as they release the torsion cause by the spring as the gearbox cycles. An AEG without bearings can get the spring to stick in a position as the piston comes back and cause the spring to break or warp, where bearings on the spring guide would release that. Plastic spring guides have a washer that lets the spring twist to some degree but are not as effective as full bearings. When installing a spring guide, make sure that the bearings will fit into the back of the piston. It should be completely inside the piston with only the flaps holding the piston out. Also, be sure to have the piston tightened all the way if a srew is used to hold it in place like with a V2 gearbox.