[RC-Monster Mike]

The Delrin spur gears hold up exceptionally well - only extremely excessive amounts of slip will cause enough heat to create a problem. we did pretty extensive testing before releasing the Delrin gears. As with any plastic spur, the margin for error is reduced(if you get a loose gear mesh, the plastic spur is the weakest link) - when gear mesh is properly maintained, the cnc'd Delrin gears have a very high load capacity. Tthey are wisper quiet as well.
Regarding the runout - while nothing is perfect, the Slipperential assembly should run pretty concentric. Some possible causes of excessive runout(if this is indeed the case) would be the spring orientation(must avoid stacking the "overlaps" on the springs), or a worn or improperly seated Teflon bushing.