Increasing effective mass will in theory cause greater problems when tracking warped records. The higher inertia of the arm in the vertical plane will cause a greater swing in effective VTF at the top and bottom of warps. How much this will effect you in reality depends on the warp tracking capabilities of your rig, and of course on your records! If your Rockport has vacuum hold-down then it's probably a non-issue.
Adding the weight on top of the headshell may be better, as Raul suggested. Not only will your cartridge/headshell interface be unaffected, but your VTA/SRA will too. If you put a shim between headshell and cartridge, you'll have to raise your arm by a similar thickness to compensate.
Read Twl's HIFI Mod thread for a very thorough discussion of the positive effects of increased eff. mass on bass and dynamics. His mod was designed for certain pivoted arms, but the principles are the same in many respects.
Adding the weight on top of the headshell may be better, as Raul suggested. Not only will your cartridge/headshell interface be unaffected, but your VTA/SRA will too. If you put a shim between headshell and cartridge, you'll have to raise your arm by a similar thickness to compensate.
Read Twl's HIFI Mod thread for a very thorough discussion of the positive effects of increased eff. mass on bass and dynamics. His mod was designed for certain pivoted arms, but the principles are the same in many respects.