My MY07 is the same but varies according to speed which is really annoying. It was out between 5% and 10% between the ranges of 60 and 120kph that I tested.
Took the measurement using 3 sources
1. GPS - internal SatNav (accessible thru the "secret menu")
2. Internal vehicle speed (accessible thru the same menu above)
3. GPS - external SatNav that I previously owned (RoadAngel)
I also thought about warranty so emailed Subaru Australia soon after I purchased the car. They were less than helpful is the only way to categorise their response. They mentioned the ADR regulations (which they actually quoted wrongly) and also said they calibrate their speedos on the "safe side" due to various states and their "zero tolerance" policy.
What they fail to understand is that my previous car (MY01 WRX) was only out by up to 3kph at 120kph which is very accurate and well within "my tolerance"

They also fail to understand that some people want accuracy over being "babied" about speed limits.
There is also the argument that because you know your speedo is inaccurate you will compensate. As such, this maybe LESS safe because you run the risk of overcompensating and you could end up speeding! For instance, I ended up adding 10kph to everything simply because it's easier to read on the speedo. However, I do not recommend this because technically you're speeding in 60 zones!
In the quest for accuracy, I purchased the Globaltop HG100. It's a bluetooth GPS device that doubles as a HUD for vehicle speed (and direction). Unfortunately the projection isn't really bright enough in daylight so you can barely make out the speed on the windscreen (mind you I also had sunnies on). I've had to angle the device close to the windscreen to give me a decent readout but this then begs the question of why do I then need to project the speed onto the windscreen when I can see the device straight on
BTW I think the ADR regs are, from Jun/Jul 2006 onwards, speedos can never underate. They are allowed to overate by 10%+4 though. So the car velocity can actually be 100kph but the speedo is allowed to read anywhere up to 114 (but never less than 100)
Cheers
Teck