Why would you want an engine that is badly suited for the marine environment. A turbo is more complex, more maintenance, more prone to failure, generates more heat, resists the kind of abuse a sailboat engine goes through less and so on. Usually, weight isn't all that critical an issue. If you have a 20,000 lb. boat, the difference in weight between a normally aspirated 75 HP diesel and a smaller 75 HP turbo diesel is meaningless. Why not just rebuild or replace the existing engine's block. Then you would not need to worry about modifying the transmission gearing, or the engine mounts.
As for your reasons:
The larger engine isn't going to give you better fuel economy necessarily. Once you're at hull speed, the amount of fuel it takes to go any faster goes up incredibly. Also, as a turbo engine, the engine won't be working very efficiently at the lower RPM ranges, and will suffer from turbo lag, as previously pointed out.
Since the engine will be a turbo, it isn't going to have much more low end torque, since the turbo requires fairly high engine RPMs to really kick in. This has also been pointed out.
The extra horsepower does you absolutely no good if the propshaft and prop can't handle it. Unless you're willing to spend the money on a larger propshaft and propellor, the additional HP isn't going to be all that useful. Most likely, the current prop would just cavitate, rather than generate any significant increase in power.
Proper propellor selection depends on many things—including the HP of the engine. I could go from a 20 HP on my boat to a 50 HP, but if I didn't change the prop, I certainly wouldn't get an increase in fuel economy or useable power—and it probably wouldn't affect my WOT speed significantly.
The advantage for extra HP is fuel economy, more low-end torque, lower rpm for normal operation, and the extra HP if needed to fight a heavy headwind, or pull off a soft grounding. I have the power curves and can send them out if anyone wants them, or you can find them on Yanmarmarine.com