Polymer composite bearings are vastly superior to any red metal alloy when they are used correctly.I agree with @NLXTACY, a brass gear against a plastic worm is a bad idea. I doubt it'll wear the worm, but it is a waste of money. What I have been working on is a bronze (not brass) ACME screw end bearing, to replace the far end plastic ACME screw retainer, held in place by two screws into the plastic. Number 1 son's power seats were wobbly, so we took them apart and I noticed that the ACME nut and the far end plastic bearing were both worn.
Unfortunately, both parts are components of the seat track assembly, and aren't listed as separate components (and aren't available anyway). I'm thinking of shimming the rubber on either side of the nut to take up that slack (instead of twisting it, as has been recommended).
The plastic end bearing can be replaced with a fender washer (modified with two mounting holes), but that's a little crude for my taste (to each his own). Since the screws are really just small lead screws (with lots of torque on them) and the near ends are supported by the motors, it makes sense that the far ends wobbling around would add to the premature wear of the near end gears (similar to the plastic end caps that keep falling out).
My evil plan is to press (or braze, I haven't decided which yet) a small bronze tube into a bronze plate, with the appropriate mounting holes and shape. My thought is the bronze will wear better against the steel screw than the plastic (which is abrasive and flimsy). It won't be much to make, and certainly a lot cheaper than brass gears. I'll post a picture when I get them fabbed up.