Upgrade cartridge or something else?

Here's a friend's turntable:

Current model Rega P3 with the RB330 arm. It has been upgraded with the Groovetracer Reference sub-platter, the Rega tungsten counterweight, the Rega white belt and the Rega Neo PSU speed controller along with a linear power supply upgrade.

The first cartridge was the Rega Elys 2. The Elys 2 was replaced by the Clearaudio Performer ($400 at that time), which was a definite improvement in sound quality.

Do you think that this table is where it should be or should the Clearaudio Performer be upgraded and if so, what would the practical dollar limit be in this case? Should he consider doing something else instead?

Thank you for your input.