Finding a 20 F and F would be much rarer than finding one IC and M. A lot of guns have had chokes opened or worse barrels cut. If it were mine I'd use spreaders like Polywad for 15/box or reload spreaders with polywad inserts. A few boxes a year would not be that hard to do. Or see if you can come across a second set of barrels to have fit to the gun. Those you could alter anyway you want and not hurt the guns value.
As to the dollar price effect it's hard to know 10-20% maybe more but you might loose a lot of potential buyers. Collectors and even some shooters put such a premium on original condition and original configuration.
