I recall reading the DA42 would do 207kt.
Obviously that was TAS.
In the end it fell tens of kt short. One could speculate it was a number of factors
- use of computer model figures
- a prototype aircraft flying at way below MTOW
- MTOW itself rising as they put in equipment which they didn't think they would need
- drag rising due to antennae etc
etc
The real question however is not whether the manufacturer of something brand new will meet the advertised specs; it is whether you want to pay $600,000 or so to be an unpaid tester
Not getting 207kt is the least of peoples' problems.