Wow... someone wants to modify their bike and people are getting upset??? What's this world coming to?!?!?!?!?!
Look... dude... bottom line... it's YOUR bike, so set it up the way YOU want it!!! Those who don't like it can suck a rotten egg cause it ain't THEIR bike!!!
Now... 21" to 17" is fairly drastic... you'll prob need to either extend the forks (EXPENSIVE) or lower the back (1/4 the price of extending the forks). Lowering the back can be done with a few new links. I'd go that route.
Now... I seen to remember (I may be wrong here) that the F800GS had an 18 inch rear wheel??? If it IS an 18 inch rear, I'd go with an 18 inch front also... it'd be a MEAN flat tracker!!!
My advice (worth as much as you've paid for it)... get some frame guards in case of a crash, and do a few track days with the 21" front. If you really enjoy it, change the front rim to the same size as the back rim, possibly with lowering links on the back to compensate.
Then get out there and enjoy YOUR bike!!!