The Ball Grid Array used in the Core i5-8350U CPU of the Thinkpad T480 is factory-soldered, and I would not even try to remove it to upgrade the CPU for any one of three choices:
1) Unsoldering and resoldering BGA CPUS is very difficult without factory-level tools
2) The chipset, also permanently soldered onto the motherboard, limits your CPU choices.
The Benicia motherboard supports many processors. Per HP documentation it should support this list of CPU:
The absolute best CPU available on this platform is the Q9650. As for speed gains, it gives a difference of about 50% faster at best.
Q9650 vs Q6600 comparison Anything else than a Q**** series processor is a downgrade for you.
Now: is it worth it? ...