Not sure what they do with the W2, or even paystubs for that matter. All this "salary should be within a few percentage points of that stated in LC... etc." have no meaning in law, when the GC is for a future employment.
It does offer them important clues in the following ways:
1. they can tell whether you changed one or more employers
2. were paid less than prevailing wage on H1 (if you were not using EAD)
3. they can speculate that you did not have a job for an extended period in between... (though the need for maintaing employment is not documented anywhere

)
etc.
If you are going to inform BCIS while using AC21, I have the following recommendations for your letter:
Group A: (Inform)
1. State that you are using AC21, and this documentation is to inform BCIS of the same.
2. State old employer name, address /end date ... new employer name, address / start date (I do not know the impact of gaps... if I did it I would try not to have any)
Group B: (Establish eligibility: state that you are establishing eligibility by the following)
3. you have a valid I-140 (attach copy) from old employer and that 180 days have passed since the filing of I-485 (attach receipt notice)
4. New employment is in 'similar occupational classification'
5. State that a new EVL is attached describing 4 above
6. Attache new EVL showing position/duties/salary and intent to employ you as a permanent ('regular' if HR objects) full-time employee upon approval of your I-485 application
7. Attach copy of valid EAD
PLUS whatever your attorney deems fit.
Good luck!