In case you don't know there are always lots of parts from Japan available for bikes sold in Japan. The most common part is exhausts. Many companies sell them along with other parts and some companies only sell exhausts. The quality ranges from OK to excellent. However the prices are almost always high.
Upgrading the exhaust will not bestow magical performance to a 4-stroke despite what the advertising might suggest. However it can give a tiny boost and they almost always sound better and often look better. They are also usually lighter than stock and this will surely be true for the GW since most go from a dual muffler to a single.
Here are some of the ones I found on Japanese sites.