Ehh, its a debate..
Personally, I find the minimal difference in performance from buying aftermarket pipes such as the FMF Fatty nowhere near worth the 250-300$, along with a silencer another couple hundred, all for such a minor difference.
(Besides, its pretty hard to spend over 550$ on a pipe and silencer and not somehow find the slightest reason to reassure yourself of the big purchase)
The stock pipes are very good these days. My policy, is that if I ever happen to break or dent my pipe, I will buy an aftermarket pipe from brands such as FMF and Pro Circuit etc.. In fact, from my knowledge, it would actually cost more money to buy the Japan made OEM pipes than an aftermarket FMF pipe, and not to mention gain that tiny bit of performance.
To tell you the truth, you should really get used to the bike STOCK for a while until you start changing it. If you fall a lot as a beginner, you could fall and damage all your new equipment that you could have put money away for later if you even still found it necessary at that time. THEN, after you get used to the feel of the bike, and you want a MUCH BIGGER gain in performance from your bike, look into cylinder porting + big bore kits - from one such as Eric Gorr. http://www.eric-gorr.com/ - A fantastic website where you can look into cylinder porting and getting a big bore, where if you have a 125cc, it can be bored out to up to 144cc! Those would be better options than aftermarket pipes IMO.
Hope that helps :)