As I explained on the other thread, anything over 2000rpm causes extreme cavitation. This gives about 6knts, but the engine will be regularly revving to 4500revs, until the prop bites again when it goes back to 2000. If I try and sit at say 3500rpm which gives about 8-9knts, the cavitiation causes revving to 6000-7000revs, and it didn't take long for the temperature warning light to come on.
Also, steerage went, with such low power and cavitation everytime you turned the engines, keeping a course became a challenge.
One idea for those with similar setups is to keep a spare cheap prop more suited to a single engine setup, in case you ever loose one engine. It could make the difference between 6knts and 15knts