I would sell or trade before chopping it down. 18.5" are barely $100, it seems like too much work to chop one down and reinstall the sight, or at least to do it right. Anyone can chop one down but to get it square, nicely cut and the sight reinstalled (centered) wouldn't be the easiest thing without the right tools/setup. Not to say there aren't plenty of people who do it though.
The 870 express I just picked up tonight is an older one with non MIM extractor and no magazine tube dimples. It is screaming for an 18" parkerized police barrel!!