I've found the way to extract great performance in the DA42-VI is to fly high (the plane, not the pilot unless over Colorado
). The rule of thumb seems to be an increase of 3 knots TAS for each 2000 feet added to the altitude.
Except when facing bad headwinds at higher altitudes, our "sweet spot" is the recommended 75% cruise power, burning 6.6 gal/hour per engine (13 gph total), and flying on O2 at around 15,000 feet. The result is usually around 180 knots TAS. This is with 2 adults, full fuel and de-ice, and luggage. Our plane seems to be at least 4 knots faster than what's published in the AFM/POH at any altitude and power setting.
Just for comparison, and for whatever it is worth, I fly my 1.7's at 75% power at 5.5 gal/hr/side and get 160 knots TAS at 15000 feet (with full tanks, 2 adults and luggage, full de-ice, and on oxygen). At 18% more full usage you get 12% more speed. Pretty good increase in speed with not that much more fuel burn. It must be related to those small but numerous airframe cleanups they did for the -VI. I find that usually I really don't save much time in going up that high unless I am on a very long (3+ hours) x-country.