Okay then, do the 21" front - 18" rear wheels
(you'll have to ask sombody else what tires are best)

Then do a Screamin' Eagle 113" (part number - 27525-08A) kit from Harley for about 3 grand and labor
I'm sure there will be some other parts too.
I don't have a Sreamin' Eagle parts book, all my book shows is the 103" kits

Send the jugs to somebody in Vegas for Diamond cut (more jaw dropping chit) like this:
http://www.diamond-heads.com/
I don't know these people it's just an example.

Detachable Chopped Tour Pak for when you ride two up

 

first - audio (speakers and amp), hidden antenna and HID headlight 6000k bulb, reclining backrest for driver, removable for passenger, and exhaust system
second - 21" fr 18" rear wheels, air cleaner/velocity stack handle/crash bars(all matching)

save all engine stuff for last (either 110 with big cam or look into supercharging)

20k will get all that and more

 

1. windshield
2. seat
3. highway pegs
4. A/C, exhaust, tuner
5. stereo
6. floorboard extensions
7. Chrome

 

Sumax does some awesome stuff in the way of diamond heads, colored engine components, and other stuff.

 

I will show you what I would do with 10K in about 2weeks when they are done with my 2010FLHX... but it will look alot like the 07 in my sig pic!

 

1. motor

2. shoes 21/18 or 21/16

3. sound system amps, 6x9's, upgrade fairing speakers, and dvd/nav/sat radio head unit

4. paint / extended fender and bags

5. chrome the h3ll out of it (bling bling)

Now thats what I would do with 20K

 

I think the best real upgrade to any Street Glide would be my garage!!

 

I'd personally trade it for a Road Glide...sorry...couldn't resist!

 

Cha-Ching!!

 

Seriously 10-20k?

Why not go out and buy another bike...maybe something from another manufacturer? Triumph Speed Triple or dare I say even a sport bike.
Having a tricked out FLHX will be great but better to have 2 nice bikes in the garage than 1 sweet bike that you will want to get rid of in a couple years for a newer model anyways.