That length mag extension wouldn't be acceptable for me, as I do not indulge in the 3-Gun sport, which is, basically, what they are for.
The only extension I use is for an extra 3 on an old beater Rem. 870 with an 18 1/2" barrel and a Streamlight, which is plenty for my usage (nocturnal varmint control), and even then, I remove it, and the light, when chasing cottontails and game birds. You get a much quicker swing/reaction time without all the extra forward weight. It would also be quite awkward for self-defense for the same reason.
As stated by OKFC05, more than three rounds loaded when hunting migratory waterfowl is a big no-no... that tricked-out shotgun would quickly belong to the DNR.