Unfortunately I(or perhaps fortunately) have never filed a screw-in choke. Most are indeed pretty hard--and many shotgun barrels are not, because they don't have to be. If an out-of-line screw-in choke is the problem, it can often be "fixed" by a custom choke. In my experience Briley can fix about any shotgun problem:
www.briley.com.On the few occasions I've modified chokes myself they were all fixed chokes on inexpensive guns. I either used a round rasp to "open" the side of the choke opposite from where it was shooting on double guns, or on single-barrels sawed the barrel off slightly to eliminate some of the choke--which often fixed the problem.
