In my experience a 2.0L DA42 will give a 165Kt TAS cruise at 12 USG (6 per side) per hour at 10,000' at 75% power. When the original DA42 did a flight from Newfoundland to Porto in Portugal it used $200 of Jet A and took 12 hrs non stop. No need for ferry tanks either if you get the extended range tanks.
I have heard various things about Diamond quality and customer service, but I presume this is a European thing as it seems quite different in the USA?