Regarding the dangerous speeds that a 2kW motor could propel you to, the full 2kW might only be used when going uphill. At 100% efficiency, 2kW will let a 75kg person travel at 10 meters/s [1] (sprinting speed) on a 15% incline.
As a cyclist, 15 deg incline is steep. Honestly, steeper than all but rural roads and even then rare. There might be some hills like that in SF. Even in the mountains, graded switchbacks are not nearly that steep, for the most part.
Which beings me to who and why would you want to ride down something so steep on a skateboard? I have a hill a mile from my house, thats ~200 vertical feet, and a around a 1/4 mile, its 12 degrees max on my GPS topo. I'm easily 35-45 mph on this, coasting. No hands descent would be NFW. 40 mph on a skateboard and there is a piece of gravel on the road, I'm not sure a Helmet will even help you with the face-plant potential.
But other than that, I think for a flat city it may be of interest. Or even a flat-ish area like soma/mission. But Manhattan? Nah. I'd pass. Maybe in the sub-urbs or exurbs? Where its just a long slog down a random road to the store or whatever would be more interesting. There is a lot of sprawl out there. And this would be cooler than a segway =D.
Yes, with stop signs =D. And Cable car Tracks. And Cross-traffic. LOL.
Bottom line = 30 degrees is sketchy, even ATGATT, on a Motorcyle.
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[Edit] Citaton: Just how steep is this?
A large avalanche in Montroc, France, in 1999, 300,000 cubic metres of snow slid on a 30° slope, achieving a speed of 100 km/h (60 mph). It killed 12 people in their chalets under 100,000 tons of snow, 5 meters (15 ft) deep. The mayor of Chamonix was convicted of second-degree murder for not evacuating the area, but received a suspended sentence.
[1] 2000 / (75 * 9.8 * sin (15))