Hi SXTY
As a TMA bod working the South sector, I'm thinking particularly of Gatwick departures going either LAM way or through SAM. My quick answer is that IMHO it will not result in fewer step climbs and MAY even have the opposite effect.
My thinking behind this is a bit objective and hard to explain, if only you were in our Ops room so it could be discussed in front of a radar!
Step climbs are sadly a necessity, and as one of the reps on the environmental group I'm aware how inefficient they are. I should emphasise that they effect aircraft of all speeds and although 'props probably have a few more, the difference will be small.
But this small difference is caused by the scenarios where, if you'll entertain my verbose explanation, is where two aircraft depart on laterally conflicting SIDS and the 'prop is the lower of the two, in front of the jet. When the prop in question is flying at a similar speed to the jet, we can entertain the option of climbing it above the jet behind as the catch up is relatively low (eg 230kts v 250kts). There are few props that this option can explored, Dash 8 D's and Saab 2000 being the obvious candidates. However, if the D's start flying significantly slower, this option may be removed.
So my opinion is that a slower airspeed will not result in fewer step climbs. Neither can I think of any reason why the simple change to all Dash 8's flying at the same speed will result in fewer step climbs.
I do hope some of this makes sense, feel free to ask for clarification if it doesn't!
Safe flying
CT