As Nick says, many feel that the standard specifications are pretty close to ideal with the '99.

Piston bearings....to my mind will sometimes improve the smoothness of the cocking stroke, but not so noticeable upon firing. I find that a polished piston coupled with an application of moly grease to the rear of the cylinder once the piston is pushed fully forwards works well enough.

Maybe interesting to try some in-depth pellet testing and see how much difference you feel different pellets make on the firing cycle?