I have only had mine out once since I had the aux on but I have the Johnson 6hp 4 stroke which is the same engine except painted white and with a different name on the outside - they are all built by Suzuki.
In a very short trial I clocked 5.5 knots on the GPS with a quick burst on full throttle (about 30 sec) but this was with a bit of a tailwind (about 15 knots on the rear quarter, not a direct tailwind) and I'm not sure what the water movement might have been. I didn't give it any longer than that because the engine isn't yet run in but it was a bit choppy which probably countered the tailwind, on flat water it may get to 6, not sure. This is on a 5.8m Humber Destroyer, one fat person (me) and not much else, main engine out of water.
Using the formula above I ought to get about 5.8 as the max. I can say one thing for sure; you don't realise how much wind a RIB catches until you're trying to control it with a little tiller steer aux engine and suddenly find you need 45 degrees on the tiller to keep the boat going in a straight line into wind
I wouldn't like to have a main engine failure offshore with 20 knots of wind blowing, but it still beats swimming
I also bought mine on the 1hp per metre rule, and also because of the light weight per HP and the external fuel input.