Just as another point of reference, I've been up to FL180 (Europe) a couple of times, but not without incident. The last time I went up that high, I did have an encounter with fuel icing. It was probably an unusual flight for two reasons - firstly it was from Alta up in the Norwegian Arctic Circle down to Helsinki in Finland, so the ambient temperature was pretty cold - about -20C all the way. Secondly, we were avoiding poor weather and were at that level for about four hours before we had any problem.
The symptoms of the encounter were that the fuel flow simply dropped to almost zero and the engine power just disappeared. Fortunately from that height you can glide for miles, so we could have easily reached an airport. Turning on the fuel pump made no difference, but shortly after passing through the freezing level at 10,000' or so, everything returned to normal. We landed and ran tests, but everything seemed fine and I haven't had a similar problem in the 300 or so hours since in that aircraft.
Reading up about it afterwards, this does seem to be quite rare. The turbocharged Mooneys go up to those levels and temperatures regularly without a problem. The only other occurrence I found had similar factors - it was an M20J, in the cold Canadian North, with prolonged exposure. The conclusion I came to was that the icing was probably small water crystals building up in the fuel pipes, which took a while to block them. The reason you might see them in the M20Js but not the Ks is that, being normally aspirated they're producing very little power - and thus excess heat - up at those levels. The fuel pipes above the engine are quite fine bore and exposed to the full blast of the icy airflow, so possibly the most likely place to encounter the problem.