Here are the differences


Xtrema 2
[ol][*]Reciever has 2 on it[*]Trigger group is different, it has a different hammer so the weight could be lightened yet hit hard enough[*]The Receiver portion of the barrel is milled out more to accomodate the new trigger group hammer[*]It has Kickoff as an option[*]The stock has been made a little slimmer[/ol]
You can use the barrel from a Xtrema 2 on a plain xtrema but not conversely. In order to do that you would also need the trigger group.

The waterproof finish is on the Xtrema I had and yes it had optima chokes. Same Choke tubes on both models