If I'm interviewing for a job, I would respond "I suggest you pay me what Fairly for what I am going to accomplish for you as a company. What my current role is, and what they pay me for, is not how we should proceed".
If they insist, I would finalize your comp negotiations before providing it.
As someone who hires people, my reason for asking them for a w2 ( and i do) is to ensure I'm not making a mistake and didn't get snowed by a good salesman...but then I'm hiring salesman.
.
An alternative might be to refuse, but given their need to validate that " you are worth it" you might need to find a way to assure them.