i can't help too much with the street legal stuff but i will be doing the same to my xc-w in early may. i would do it now but im going to baja in just under a month and don't like the idea of possibly not getting my CO title in hand b4 the trip. i have licensed dirtbikes in the past but i think they've instituted some new laws since like the vin verification with troopers only. used to be you could get the local boys to sign off on it.
i do like running a trials tire for a few real tech trips throughout the summer. i know the pirelli is dot approved so that takes care of the rear but what r u going to put on the front? i dont want to blow $75 on a tire i will never actually use except for the licensing. maybe the front i settle on for baja will be dot approved and i could use that.
as for the protective stuff for the bike again i would put the Hyde Racing skidplate on. that material and mount is bulletproof.
for the seat i've swapped the stocker for the enduro engineering tall soft seat. i'm only 5'11" so the tall seems like a bad idea but in reality it sinks pretty well when you sit on it and it is very comfy. this seat also holds me in position very well as it molds around your butt (which is great as long as you start off in the right position!) EE makes a normal height soft seat but it is a different animal and not nearly as soft.
regarding the tame low end you are talking about for tech riding you might find the map switch to work well for the least $$. (providing it actually works on our bike. my buddy had one on his 500 and swore he could feel the difference but took it off as he was always running stock setting anyhow).
i need a horn but want it to be small and inexpensive (good luck, i know) as i prob wont run it often. i'd be interested in what you decide on. also the brake light switch, i've had bad luck with the rear brake/banjo bolt type switches breaking and losing pressure. i've heard there are switches avail for the front brake, any experience?
the jd tuner sounds like a decent buy but i havent found a reason to blow the money as i couldn't imagine needing more/richer power. Is anyone running one?? results??
i would like the shorai battery as well but the stocker is so strong and the bike starts immediately when i hit the magic button, maybe next winter. the shorai website says to turn something on the bike on for a little time during cold weather starting b4 hitting the starter and it will perform much better. but my headlight doesn't work with the bike off sooo.. maybe kick bike first time on cold mornings?