Looking at a fox rp3 pushed shock by tft second hand, no real understanding of how they work? and being 13.5st whilst the current owner is 11.5 st does this make any difference? and then what air pressure do you put in?

pushing custom valves the shock to rider weight, bike (single pivot/vpp/maestro etc) and also riding style

so for it to be good for you you must have teh same setup, same weight etc

Sounds like it might not be a good idea with a two stone weight difference.

But you could call TF Tuned and ask about the cost to modify, especially if you think it would need a service anyway.