effective mass of rb300 with hvywght, , Lyra clavi

I bought a used Rega Planar3 with an rb300 arm, and I am searching for a cartridge for it. The arm is has the express Machining Heavyweight counterweight upgrade, and I am curious how that effects the effective mass of the rb300, which is 13 grams without the counterweight upgrade.
I am also thinking about picking up a slightly used Lyra Clavis from my dealer; any opinions on that cartridge? I believe it isn't the Evolve99, just the Clavis da Capo. The other option I have been looking at is a Benz Micro Ace (the medium output version), or possibly one of the Shelters.

Associated equipment:
Rega Planar 3, with rb300 tonearm
BAT VK 3iX Preamp with phono card
BAT VK 220 amp
Gallo Nuclear reference 3 speakers
Kimber KCAG interconnects
Kimber 4tc speaker wire