by what I've read the Gnarly pipe isn't very good on the 200. The Fatty is supposed to be better but I don't think FMF makes the Fatty for the 200 anymore.
for reference/food for thought, my parts manual shows the 200 and 300 use the same stock pipe (2004 year). The 250 and 125 have there own pipes. Yet FMF only has one Gnarly pipe for the 250/300 (either or) per their web site.
I have a Fatty on my 200 and low end is so so, mid is real nice and top end screams. I do have a stock gas gas 200/300 pipe but have not tried it yet to see how it compares. Still working on jetting and such with the current setup. Don't want to change pipes now, I'd be all messed up.
Personal I don't like fmf stuff. Pro circuit and bills pipes have always worked great for me. fmf always felt....flat.... or just top end pipes. To bad there's not much to choose from for the gas gas.