I've never noticed any patterning problems when cutting a barrel down. I have not gone below 18.5" though. You loose the choke and some poorly made guns the core of the pattern may shift slightly if the bore was not centered. But nothing I would call a problem. You get cylinder choke but if there's enough barrel thickness that could be jug choked or vancomped
I dont think these guys do anything that would be considered a cheap alteration and with their reputation I cant see there being any issues.
Seems like a lot of work to loose what 2.5" of barrel? Looks good though