I have a P229 .40, and a caliber exchange kit for a .22
As a .40 it is unmatched for accuracy, and feeds any ammo.
Sometimes I replace the barrel with the .357 Sig, and it works very well. It has the same point of aim which is nice.
I have only run a few boxes of ammo through the .22 exchange, but did not have any problems with different ammo types.
The best enhancement by far was sending it to Sig for an "Action Enhancement". It was a good pistol before the mod, but now it really a great pistol in all calibers.
I cannot comment on the P220 since I don't have one, but I like it better than the P226 TACOPS that I have.
Last year Sig ran a special on the caliber exchange kits for $219.00 and that is a bargain. I keep looking for it again this year but have not seen it.