Pretty accurate (102 at 100 or so). True, but sitting on 100 doesn't neccessarily mean watching the speedo constantly. I know the noise/vibration my car makes at 100 and 110, and even if I've got the music on, I glance down every 10 seconds or so. Far from watching it like a hawk.
I know it sucks, but if your speedo on the Stagea is out, cops aren't gonna care too much... They'll tell you to get it fixed. Probably not a bad idea anyway; that way you can be like, bam, I got it fixed recently, here's the receipt and calibration data. It said this, and no more.
I dunno, it's an iffy one. All we can do is try. The thing is, they generally sit in the same spots all the time, so if you're coming up to where one is, just slow a little bit, and if someone behind you gets pissed off, they can go eat a dick.
Having said that, sitting at the bottom of hills isn't cool, or halfway up but obscured so you don't see them till the very bottom... So it's a toughie. On flat road it should be fair easy to maintain speed, but downhill sucks, I hate riding my brakes/4th gear.