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.
The usual plane salesmen's trick
I've been up to FL200 in the TB20 and can tell you that breathing at FL180+ is a full time job with a cannula. It's OK but you need to monitor passengers fairly carefully. Masks are fine but you get through an awful lot of gas.
I wonder what the ceiling is. That is really more important. If it does 147kt IAS at FL180 then it should go to at least FL250 and that is a really useful capability because you can outclimb most warm front weather, and you can outclimb most organised IMC in which nasties could be hiding.
Most turbocharged piston IFR tourers will do FL250 or so but almost none of them make TBO without changing some cylinders (or worse).
But then the Diamond engines are much less proven than that, so we have some way to go.