I personally think that 20mph would be a perfect cut off speed for the speed restrictors fitted to EU spec'd pedelecs. I can easily hit 28mph rolling down the Wells Rd in Bristol and fear if I came off at that speed i'd likely hurt myself badly even if I didn't roll underneath something, or hit a lamp post etc.
That said, the 17mph of my Kalkhoff Pro connect is perfectly good for getting around town without freaking out the motorists (who are used to expecting a cyclists to be doing a few MPH less than that on the whole).
30mph is a serious speed on a pedal bike if you're using them on the public roads, as well as a serious battery drainer. I love a bit of speed but the more i've thought about it, the less the idea of hooning around at 30mph appeals to me.