The difference in revs is mostly down to gearing. The Alpha 1 sterndrive with the V6 had a 1.63:1 reduction ratio, whereas I would be surprised if yours was not 2.00:1.
At 20mph, about the minimum comfortable planing speed on most sports boats, the V6 Maxum was turning about 2,400rpm but my previous Monterey 3.0L would be doing about 3,000rpm at the same speed (with the same propeller). This is simply a gearing issue as 2,400rpm / 1.63 * 2.00 = ~3,000rpm. Incidentally, at 3,000rpm the 3.0L can produce a maximum of around 100hp, whereas at 2,400rpm the V6 could make up to around 120hp. This is not just a coincidence, as the gearing is selected primarily to ensure that the engine is capable of producing sufficient horsepower to propel the boat. At 2,400rpm the 3.0L can only produce around 80hp, which might simply be inadequate to propel the boat at 20mph, or get it out of the hole and onto the plane in the first place.
Flat out, my Monterey would do just over 40mph at 4,800rpm WOT, where it would be producing its maximum 135hp. The V6 could push the larger Maxum at 40mph at only around 3,800rpm, where it was capable of producing up to around 190hp. The V6 was therefore being worked relatively less hard both in terms of revs and torque output - this is why larger engines (in boats at least) do not necessarily use much more fuel than smaller ones. This is especially true where the motor is frequently being worked hard, such as when towing wakeboarders!
This 'spare capacity' of larger engines tends to let them run relatively more efficiently, dropping the sweet spot a little lower down the rev range. This, combined with the significant gearing difference, is why the sweet spot of your boat is that much higher up the rev range. Increasing the engine size in any given boat generally tends to reduce the revs at the sweet spot but the dynamics of the hull usually come into play here too - the hulls of many sports boat seem to be optimised for a best cruise speed at around 25-30mph.