Basically, inlet manifold vacuum causes extra air to be sucked in around the worn shafts. The amount that's sucked in varies with the rotation of the shaft. It also varies between each carb. When adjusting the mixture at tickover the jet needs to be lowered to increase the amount of fuel needed to give the right ratio . When extra air is leaking in, the jet needs to be dropped further to compensate. This means that extra fuel is being added regardless of the throttle position. If the needle is correct for the application, the fuel/air ratio should be close to correct from tickover to full throttle. Fine if you've no leaks, but the addition of a varying amount of extra air will weaken the mixture across the range. The temptation then, is if the engine shows signs of running weak, is to richen the mixture more. This reduces the mpg even further.
It's rarely mentioned that leakage around the shafts does reduce the vacuum available at the jet. The SU's working principle is entirely related to vacuum. Any reduction of the vacuum before the butterflies will have some effect. The piston must be matched to the dome to give the closest fit possible. It's virtually an interference fit. The interference is made by the air having to fight it's way past the grooves around the piston. These grooves break up the air flow and cause a controlled seal. Think of the front edge of a wave rolling up the beach. It might seem a good idea to take a bit of metal polish to the insides of the dome and the piston edge but this will destroy this interface and reduce the ability of the vacuum to lift the piston. Petrol soaked rag or carb cleaner only. No harsh rubbing. Both carbs should have a dome/piston with a similar efficiency so that they open as a pair. Just loosening the dome hold down screws and making sure that the pistons rise smoothly (dampers out) and return at the same speed is worthwhile. You can also try it without the springs if you think they move differently.
The carb body needs to be line bored and reamed to the correct matching size for the new shafts. Best left to the pros unless you're happy around precision tools. Burlens will supply a kit plus the matching reamer if you want to try but the reamer is quite costly, so cheaper in the long run to have them done by one of the shops recommended here.