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
I believe that the RB300 is actually about 11 grams effective mass.

I think that the standard RB300 will work fine with the Lyra, or the Benz, and would need a bit of additional mass for the Shelter. Some people use the DIY HiFi mod to very good effect for this.
Dear friend: You can do better through a Sumiko Celebration, this one is an exllent cartridge and hard to beat for less than 3K.

You can find one of this cartridge here in the classifieds for only 800.00. Don't miss it.

Regards and enjoy the music.
Thanks for the responses; I am still wondering about the effect of the heavy counterweight on the effective mass of the tonearm. Does anyone have any idea of the change with this?
Jwinder, the counterweight is meant to counterbalance the cartridge on a statically balanced arm such as the RB300. If a heavier counterweight is used, then it will need to be set closer to the pivot than a lighter counterweight would, in order to counterbalance the cartridge. Since the effective mass calculation is based on mass times distance(from the pivot) squared, having a heavier counterweight located closer to the pivot would cause a DECREASE in the effective mass of the tonearm, due to the exponential nature of the equation.

So, if the Heavyweight counterweight is, in fact, heavier than the stock counterweight, then it will decrease the effective mass, compared with the stock counterweight. This will result in a change of the mass/resonance of the system, and also will reduce the moment of inertia of the tonarm and thus will improve speed of warp tracking. Whether this will be better or worse for your particular application will need to be determined by calculation and by listening.
Thanks for spelling that out clearly, Twl. I don't have the original counterweight, since I purchased the Rega used, but I will be able to audition the Clavis cartridge to check compatibility. Incidentally, the 13 gram figure for the effective weight was pulled off of the official Rega site; presuming that the counterweight is heavier than the original one, I would guess that the effective mass may end up at 11 grams or so, if the figure on their site for the regular counterweight is accurate.

When I put the heavyweight on my arm all I had to do was move it further to the back of the arm stub from where the stock weight was.

Don't discount the woooden Grados on that arm and table. I have owned the Platinum on up to the Reference that I use now and the results have always been great.

Its as if the grados were made for that arm and table as no vta adjustment is needed the setup simple and the sound is always great.