A lot depends on how much business the mortgage broker places with the lender with whom s/he is haggling. A lender is unlikely to offer special discounts to a broker who places little business with them. They are more likely to accede to an occasional request for a special offer (within reason) from a broker who places tens of millions of mortgage business with them per year.
It also depends on the application. If the applicant only barely qualifies for the requested loan, the lender is less likely to offer special terms. If the loan is small, ditto.