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I don't know much about boats and am under the impression that anything with an outboard motor is an entry level lower priced example. Anything high end and luxurious will have the engine mounted inside the boat, not hanging off the tail end. So what's the point of this 1200 pound beast? Wouldn't anything worthy of that much power have inboard motors?
As far as is a 600 hp enough, that same question was probably asked when 300 hp engines came out. If there’s a market for it, it will be made.
It gets quite costly to build a boat with a strong enough transom to support these engines versus a boat with the engine inside of it.
Also keep in mind an outboard engine is made for its intended purpose, an inboard/outboard engine is a modified automobile engine block.