As an alternate, consider castable polyurethanes. These are not plastics, but thermosets and offer good abrasion resistance, self lubricating in water and corrosion resistant in this environment. They are available in 70D or harder and additives are possible. In fact, UHMW chips are blended...
Check the following site for soft, castable polyurethanes. They have consistent properties, batch to batch, and tear strength values in the range of 14 pli. www.crosslinktech.com
Check plastic distributors such as Cadillac Plastic or Johnson Plastics. Their information sheets usually show a heat distortion temperature for guidance.
You may want to consider a 70D ether based castable polyurethane. Parts made in this material will exhibit the properties your after, but material costs are high. Tooling costs on the other hand should be far less.. depending on shape and processing method... quantities etc.
The coefficient of friction for rubber involves very complicated processes and I'm not sure that anyone really knows. The surface wetness and roughness, type and hardness of rubber, type of surface, dynamic or static.... too many variables. I'm sure you've heard all this.... You almost have to...
The shape factor is best effected by adding or removing shims. In this way you can better modify stiffness. External restraint to limit bulge is not the best approach. I would expect cutting action on the rubber surface which would probably be detrimental to bearing operation and life.
