I wasn't aware that they change the trigger mechanism when they install the nitro piston. I would assume that if they are going to change the trigger to the metal one then they would have to change out the entire trigger group, but then again I haven't had the opportunity to personally inspect one of the new GAMO trigger groups so perhaps I'm wrong. I'm sure one of the members who knows for sure will chime in.
As for the Whisper .22 ,,,, you've already made the order so I would say don't second guess yourself. Get the rifle, shoot it and see how you like it. If it works for you then great, if not then you can always return it to P.A. and order something else.

As for the XL,,, yes,, they are a heavy rifle but the sling works great for those long walks in the woods. I love mine and just finished doing the final sight in with it. Just waiting for some nice weather so I can take her hunting.
Another option you could look into if it winds up that you don't like the whisper .22 is the standard Trail NP (not the XL). It's lighter and much easier to cock and designed more for hunting AND plinking,,,,, where as the the XL is definitely not designed to target shoot with.
Let us know how it turns out for ya with that Whisper....
Jeff