Like you, 2-300 mile days are my norm, and occasional 4-500 mile days.
Seat: I use a "Spencerized" seat, (
http://www.greatdaytoride.com). For a very reasonable amount, Spencer will mod your seat, and it DOES add miles per day to the bike. On longer hot rides, I use a beaded seat cover. In winter, I sometimes use a sheepskin seat cover as well.
GPS mount: Unless you always ride areas you know ( what fun would THAT be?), you'll likely want to mount a GPS. I tried lots of mounting positions, and ended up mounting Ram balls to the top of the dash, on an aluminum plate. It puts the GPS or iPhone in a very easy to glance at location. I have a "cigarette lighter" style of 12v outlet in my left fairing pocket.
Top Boxes: This is always a lively ( read
pinionated) discussion. I love my BESTEM, and it has many miles on it. No, it is not as nice as a Givi, but for less than the cost of a GIVI mounting plate, you can have a Bestem with LED brake and running lights. It'll hold 2 helmets, ( of most types, ), is easily detachable, and makes commuting or touring MUCH easier. Get a set of hard bag liners if you are going on a multi day tour.
Back rest for a pillion: If you have a top box, you won't need this, but my Pillion HAS to have a backrest.