I upgraded calipers on my 02 156 2.0TS with 4 pot Brembo calipers and 305 rotors from 166, I believe they are quite a bit cheaper than a set from 159 or Mito. A set of 305 rotors from 166 go on the 156 without any modification, but the calipers need an adaptor bracket which looks something like this
My mechanic order it from a local machine shop if I remember correctly it cost around 65USD(labor is cheap where I live:rofl

I don't know all the exact details because I didn't order this kit directly, it was taken off another 156TS when the owner converted it to a GTA. I'm visiting my mechanic tomorrow, so I'll ask him the details.
Here are images of my car