it should be quite obvious to anyone that looks at an 8.5" Grizzly that it is a gun designed to fit a
niche: to be as compact as possible while still maintaining the 660mm non-restricted OAL. only an idiot (or someone with a twisted sense of humor) would buy one for waterfowling.
if you want the most compact pump shotgun that still maintains full shotgun functionality (ie: not a useless PGO) you have 2 choices:
you can either use an 18-18.5" barrel and a folding stock (18" 870P barrel pictured)
or
you can use an 8.5" barrel and a standard stock
the 8.5" has the advantage of being lighter, shorter, less bulky, more durable and the stock
does not need to be deployed.
the 18" has the advantage of having a higher capacity, which is useful if you are LEO or MIL but irrelevant for Canadian home and wildlife defense scenarios.
so you have two choices: you can either chop/fold the stock, or you can chop the barrel. personally i do not consider a shotgun fully functional unless it has a full stock, therefore id rather sacrifice the barrel end.
btw: patterning has little to do with barrel length and a lot more to do with your choke - or lack of it in the case of these short shotties. if you are concerned about your patterns then take the gun to your gunsmith and have it tapped for Remchokes.