Mine shows exactly the correct speed, but my buddies is 5 mph off. I don't think they are all the same, tire wear, air pressure, etc. can make a bit of difference in actual speed and indicated speed.
You've got a rare bike there, indeed.
Mine's 4-5 mph off, depending of course on the tires I'm running at the time. I check EVERY vehicle I own against my GPS to find out how accurate the speedo is and also checking the ODO too.
My GoldWing is actually the first bike I've ever owned that's actually close. It reads 1mph faster than I'm actually going.
My V Strom's the worst I've ever owned, with it reading 7mph faster than I'm actually going.
My KLR's in between, at 4mph off.
The weird thing is, with these speedo readings off this far, the ODO is usually within 1-2 miles per every 100..
For your street set of tires, definately look into the Michelin Anakee line for great street grip and long wear. When inflated correctly, you'll see almost 3 times the usage as the typical 50/50 tire.