The Gearbox is the biggest improvement with the '09 KWA's. There is also their 2G hopup buckings which are a phenomenal improvement over the ones in the '08 models. The gearbox you are referring to in the later models of the '08's is what KWA called their 2.5 gearbox which was kind of a teaser to the new 2 GX. For all the info straight from them pop over to the KWA forum:
http://www.kwausa.com/forums and read till you can't read anymore.