Bumpy, it's possible that for your (rider) weight you don't have enough preload and damping at the rear. Given the info you've provided the front suspension may be not behaving quite right because of a rearward weight bias (not enough rear preload) and then pitching to the front (not enough rear rebound damping). In fact without enough rear preload your bike may not be steering as precisely as it can either.
The damping qualities of the OEM units are of reasonable quality, they are not shite as some claim, and more than up to the job for most riders on UK roads. It's the spring rates that are not quite right but it's often the entire assembly that gets the blame. 2004 and 2005 models came from the factory with White Power suspension front and rear. From 2006 they've come with Showa units. The rear tends to be a bit undersprung and the front slightly oversprung.
Unless you really want to splash out hundreds of pounds have a serious look at Hyperpro replacement springs. They do recommend replacing them as a set and I believe that's good advice. The original springs on my '05 GS (White Power) were are rated at 50 NM initial on the front and 130 NM on the rear. Hyperpro progressive springs for the White Power units are 47 NM front and 147 NM rear. After installation the bike was transformed, particularly when riding with pillion and panniers. And it was a heck of a lot less expensive than installing premium units. The front is noticeably plusher and the rear a bit firmer as the spring compresses, which you would expect given a progressive spring. I'm 75 kg. The damping qualities at both ends are just fine. The rear rebound adjustment screw is out 1 1/2 turns. If you go this route make sure to top up the rear's preload adjuster reservoir (easy) when the spring is off.
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) - although they're usually one and the same place, IYSWIM.
