200kt TAS at FL180, ISA, is only about 147kt IAS
Well, except that prop efficiency falls off with altitude. My plane will do 140 KIAS at sea level (well, a few feet above it works better) but at FL200 it's down to under 125. Maybe that's not true for all props/planes.
I agree that these kind of altitudes really require pressurization. I've done long flights at FL180 and above, a handful of times, and you spend a LOT of time checking the oximeter and the integrity of the plumbing! I very rarely go much above 10000' unless I need to for the terrain.
All I really want is something which doesn't actually go SLOWER than my TR182.