For every "proof" there is someone/some thing that will "disprove" it. It is support, not proof in my opinion, for whatever that is worth. I believe what they are looking for is support for the opinion, not irrefutable proof, since that seems impossible when humans buy with emotions. Doubt anyone is going to prove anything, nor that the GSEs expect proof. Question comes to what is support then. Is it "market reaction" without any information on how that was extracted? Is it data showing how one arrived at an adjustment, such as the spreadsheet showing regression analysis? Might be different for different underwriters of course, but my suspicion is that major elements of comparison that lack bracketing might cause the question of support, or adjustments that widen the unadjusted range might trigger that question.