Realistic TAS for the Q400 is around 315-325kts, depending on weight and conditions. ATR -600 wouldn't be much slower than that.
QLink operates the Q400 at these speeds for whatever reason, it can easily achieve 340kts in cruise.
The ATR-72-500 is only capable of 275TAS at max weight in optimum conditions. The -600 has 5% more power available for take-off and max continuous climb. This allows an increase in max take-off weight and cruise altitude capability but there is no mention of increased speed as the cruise power settings would not increase much. Considering the increased weight the aircraft should be about the same speed. The ATR 42 because of its lighter weight and smaller frame is capable of 300KTAS.
The speed difference on the longer sectors is noticed by the passengers, even when its only 40kts its still around 5-10minutes on a 200nm sector and the slower time to climb will also make a difference.
Other differences between the two aircraft is that the ATR 72 will struggle above FL160 heavy (especially in ice), the dash easily makes FL200+. A big consideration with weather in SE Aus. During winter the worst icing occurs in the 10000ft to FL200 range. At least a SAAB will happily fly long distances under 10000ft with its helicopter engines to avoid headwinds and ice.
The first delivery of ATR-600 to a leasing company is not due until late 2011.