Take what I am about to say with a pinch of salt, as while from my experience W/KG scales well for performance from 7" to 42", and I fully expect it to scale to 6', 42" is my biggest boat and I have no first hand experience of anything close to 6'.
Firstly sorry to offend you if I am stating the obvious here, but I have to ask as you are talking about your boat's weight and asking for purchasing recommendations for motors and batteries (so I am assuming that neither are in the boat at this point). You know W/lb is based on the RTR weight, not the weight of an empty hull right?
You can run 150W/lb in an FE boat, but I would consider that a minimum, I personally run 500W/KG (about 230W/lb) in most of my boats which are pretty efficient racing designs, and I think the typical boat on here is running considerably more than me.
You certainly won't be getting anywhere near 78mph on 150W/lb. I haven't seen your hull, but I would guess at the mid 40mph range from a typical scale offshore cat on that power. I expect abut 55mph from a flat ride pad stepped race cat at 500W/KG.
Motor and ESC power specs are the highest power they think their product can take without blowing up, which is generally well past its maximum efficiency range, I generally run my motors and ESCs at about half their quoted maximum power for more efficiency, cooler running and a longer lifespan.
Paul Upton-Taylor, Greased Weasel Racing.
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