My 590A1 currently has the following modifications:
Davis SpeedFeed stock with the edges on the buttpad rounded to reduce snagging on clothing.
NON-QD rear sling swivel
Scattergun Tech +1 extention and side-sling adaptor, for Rem 870, modified to fit the M590A1
Vang Comp hot pink anti-bind follower
SureFire #623F (constant on option) with the rear half of the pressure switch blocked
Ashley Outdoor (now XO Sights) Tritium front sight, thru Vang Comp
i'd like to get the internal modifications to the barrel by Vang Comp.. perhaps that'll be in the budget someday.