First thing, if you make a lot of changes to a P-line to reduce its weight, you will end up with a total price exceeding the one of the T-line -> better to buy immediately a T-line.

Limited changes to reduce the weight are the saddle (for something like an AX-lightness Leaf) and the brake levers + brake calipers for instance for Ridea levers and calipers or better (but much more expensive and more difficult to do) for eeBrake calipers.