The entire assembly of the P320 fire control unit is even less remotely similar to the internals of the DA/SA SIG hammer-fired Legions than the trigger shoes are. I mean, the only way in which there is any similarity whatsoever is that there is this curved, stick like thing that when you pull on it the pistol goes bang. The fire control unit of the striker-fired P320 does not have one single part that is interchangeable or even similar to the internals of the classic P series pistols.
For your information, the P320 has had three different triggers since its introduction. The original trigger shoe had a thicker profile than the current trigger when looking at it from the side but was hollowed out on its backside all the way to the tip. The original trigger shoe also left an opening in front of the shoe exposing the bottom of the grip module. This trigger caused trigger finger sting in some shooters and was modified by SIG.
The second iteration of P320 trigger shoe was called the "adverse" trigger. It had the same lateral profile as the original but added a "mud flap" to the front of the shoe that served to protect the inside of the grip module from dirt entry. The adverse trigger was also "filled in" on the back at the tip. This trigger seemed to solve the trigger sting issue but added some mass to the original trigger.
Then came the drop-fire issue. Among other modifications including the addition of a disconnector and a change in sear geometry, SIG reduced the mass of the trigger shoe in order to reduce its inertia. The newest P320 trigger, the "low mass" trigger shoe, is much thinner in profile than the first two and much thinner in profile than the Legion DA/SA triggers. It is once again hollowed out on the back side all the way to the tip. I have experienced some trigger finger sting with this new trigger but inconsistently so, and I have heard of a few other individuals who have had the same experience.