Don't ask me how I know, but the satnav registered 137mph when the speedo was touching 150, so it seems quite a way out. Not sure how typical that is.
This is the link to the English summary of Christian's web page.
http://cvieth.bplaced.net/ganganzeige_summary.html
To place an order just email him: cvieth@vodafone.de
My experience is that most vehicles on the road today have speed readings that are overstating reality by as much as 10%. So your GPS measurement of 137 compared to your speedo's 150 is about right.
(perhaps this also has something to do with the 'margin' that the traffic dept allows you before the speed camera takes your pic ... but that's a different topic all together ...)
BTW, I have the multiguage fitted in mine (lucky enough to come with the bike) ... Fantastic piece of kit !!!
Having read through the installation instructions & the operating instructions, I reckon it should be quite easy to adjust for the correct speedo reading. The difficult bit (actually not 'difficult', just labour intensive) will be to solder the wires in the correct places ... but its all well detailed in the docs.