6hp Johnson 4 stroke on 5.8m Humber Destroyer about 4.5 knots
4hp Merc Sailmate 4 stroke with high thrust prop on Osprey Vipermax 5.8 about the same.
Likewise not much difference with leg up or down, less than 0.5kt anyway and much easier to steer!
Actual GPS speed generally fluctuates between about 3.5 and 6 knots over waves etc.
I find that about half throttle on the aux works best, full throttle gives very little more speed just more noise/vibration and higher fuel consumption.
Another useless bit of information is that with both the aux and the main engine running you can pretend you have twins and spin the boat on the spot