Here is his story http://www.gruntsbushings.com/index.html
The E23/E24/E31/E32/E34 and I believe some of the earlier models all use the same bushing sizes.
The material has been changed so that the E32 750 bushings are the standard bushings of choice even though the E34 M5 bushing is technically stiffer.
Arm shapes and lengths are different on some that use the same bushings.
The E32 and E31 share arms but the E31 has a solid bearing instead of a rubber bushing in the lower forged aluminum arm.
Poly bushings have a shorter life and a harder ride with more transmitted road noise.
Grunts are noted for lasting longer than the other poly bushings on the market.
Ball joint replacement requires a new arm because they are not replaceable by themselves. Poly would be a poor choice even if it were available. Ball joints are metal on metal with a lubricant in between to prevent wear.
When I replace my Grunts I will probably use the M5 bushings to cut down on transmitted road noise. I don't drive the 840 hard enough to justify the exta stiffness.