My '07 R12R reads 5MPH fast at 60MPH, and close to 8MPH fast at 75MPH. BMW once wrote up their allowable error margin, and it's a formula that includes a simple MPH error and a % of MPH error. It is always a positive error (reads higher then real speed.) The claim made at the time was they were trying to avoid ever having a speedo that read slow. My Porsche speedo agrees within 1MPH with my GPS... so it can be done and the Germans know how to do it.
I've always relied on my GPS for accurate speed on the bike. Anytime I go past one of the radar speed warning signs the GPS has been in perfect agreement with the sign, the speedo exhibits it's usual error. This is true of many other reports from other riders when we've discussed this.
Know how in a BMW car passengers will say "It doesn't feel like it's going 100mph.."? That's because it's actually doing somewhere around 90 (currently own 1 BMW car, and have owned many of them.)