Some lose up to 2 mph.
If you are pushing hp's in the range to where an XR gearset is already smearing the gear faces under load, then the BravoXR is actually sucking more hp thru localized friction than the Bmax will. In that case there will be no speed loss with the Bmax.
I have a friend who is involved in a three boat trade-thru. The potential owner of his current boat is planning to do a power swap on it. This will leave a pair of 575SCi's and Bmax drives needing a home. Probably can be bought very reasonably. If you have interest in the motors or drives, PM. The complete construction of the deal is not complete, and buyers for those components can help to make it all work. Motors and drives are still in boat and running so you can see what you're getting.