Fin, sail or board? What is the most important for going fast? Sure everything has to be reasonably capable of going fast but what will make a difference if you are already going reasonably fast. I would think it would be the fin.

that is the beauty of windsurfing for you.
you can't single out one aspect of the whole system for the silver bullet as your speed will only be as good as the weakest link.

you can have the best boards/fins/sails in the world, but if only for example the sail trim is off you will never beat any records.
windsurfing is all about fine tuning the kit you have and constantly work to find the right balance.

as for best bang for your buck gear improvements i would say go for fins
...of course i would say that... ;-)
they produce the counter force to the whole rig, but are considerably cheaper to play around with and try new ones compared to boards or rigs.

Boogie
www.C3-fins.com

Guess it depends on what your weakest link is.
As boogie says, the weakest link is what's restricting your speeds, (if you have optimised every thing else).

"Flat water" you could get any kit to go faster without changing a thing.

I'M WITH BOOGIE, ONCE U HAVE EVERYTHING SORTED AND BALANCED.
I NEEDED TO FEEL THAT SENCE OF SECURITY IN THE CONDITIONS I'M SAILING IN,
THAT COME FROM MY FINS
TIME ON THE WATER EXPERIMENTING WITH THE GEAR YOU HAVE, AND YES CHOCO
FLAT WATER THATS WHAT GETS YOU THERE EVERY TIME.
OH AND WIND !!!
3 WEEKS OF THIS WILL NOT MAKE YA GO FASTER !!

Choco is right on the money, flat water makes all the difference, when I got my missile I did a 40 on the first day just plugged in a 5 koncept and didn't adjust one thing just sailed, had the wrong mast and straps fully forward but the water over the weed beds was flat as could just rake it back and hold it there.
I think choco has done over 40 everyime he comes down here to my local.
I am very lucky i have a great flat water spot in my backyard.

Yep the whole is greater than the sum of the parts....and then there's flat water :)