Usually lean mixture gives more power, cars run on the rich side for safety as it prevents detonation and pistons melting (very lean mixture )
Front sensors are closed loop unless running WOT so it's these that tell the ECU how much fuel to add, post cat sensors are basically checking the ECU is working and the cat is doing its job
I think I paid £350 for 1 post cat sensor replacement at main dealer, that included initial diagnostic and code reset, I would do it myself next time and only paid as I had not had the car long and wanted it fully checked out for piece of mind.
