The Q400 has a cruise speed of 360kts, and on flightradar24 most Q400s that I looked at reached similar speeds during cruise. The ATR 72 has similar specifications yet only flies at 280kts. I'm currently designing a regional turboprop as part of a university assignment and I can't find a drag polar that gives 360kts as anywhere near the optimum speed for L/D ratio, 244kts was the cruise speed for my initial configuration. Is the Q400 likely flying at a much higher, less efficient speed? Why would this be done? Or has better technology allowed it to fly efficiently at this speed?
Edit: According to a pilot on this flight sim forum they used to cruise at around 200kts. This video shows Flybe (budget airliner) cruising at 240kts, FL240. Maybe the flights I've seen on flight radar are operating under different parameters at the moment, with fewer passengers and lower oil prices maybe 360kts is the best speed.