Mod my Rega P5 OR...time to move on ??

Adding mass is where you are likely to see the most improvement but unfortunately cannot realistically be "upgraded". 

Regards

It sounds like you have the upgrade bug, and typically a heavy platter table will give you net gains across the board if well designed. Selling your Rega and finding, for example a used heavy Systemdek would be a reliable pathway, among others of similar design.  Cartridge tonearm compatibility is a factor also. I would stay away from anything "vintage" as it's not necessary and there can be downsides. There are high value new tables available also. The better tables are more highly resolving and better reveal layering and transient fidelity not to mention frequency response  and an overall more even, sophisticated sound. Your ZYX is a very  fine cart also. 

Substantial improvement is possible without an unnecessarily large outlay.

I'm not sure you will notice as much of a difference moving up the Rega line as going to a heavy platter design. Maybe just stick with what you have unless you are finding a particular parameter lacking. 

As a budget alternative I have used ZYX carts in Jelco 750 tonearm and the carts work quite well with the heavier arm.