200kt TAS at FL180, ISA, is only about 147kt IAS.
The stock DA42 TDi will do that much at low levels so it's just a case of having enough turbo boost at FL180.
With constant power, IAS declines with increasing density altitude. TAS does increase since drag decreases, but to keep IAS constant would take more power, since IAS is proportional to the drag and power = force x distance / time, ie. at a given IAS it takes a certain energy to cover 1nm through the air. At higher level and higher TAS, that happens in less time, so energy expended per unit time (power) must be higher. IIRC, prop efficiency is a minor effect. As a very rough example, in the 421 at 60% power, IAS is 165 at SL and 140 at 20k' but TAS is 200kts.
Many turbo piston engines maintain SL boost well above 15k, to >20k