Shadrach, thanks for starting this topic. I myself was planning on purchasing a shotgun in the near future and was considering one of these for essentially the same reasons you were.
As far as accuracy with slugs and buckshot is concerned, how will the 18" barrel hold up to a 20" M870 with rifle sights or 22" Win 1300 with rifle sights? Will the barrel length and/or sight radius have a significant impact?
In CDNN Investment's
latest catalog, they are offering the FN Police with a SpeedFeed buttstock for $270 (see pg 35).
If you are planning to order through CDNN, I would recommend getting on their mailing list for the promotions they run. Currently they are offering free shipping for orders over $75 with the code "fs75."
ETA: It looks like at least one person was able to put a mag extension on their 1300 without changing barrels:
GunsAmerica link.