A prop upgrade is definitely called for. An 18" pitch is not an "upgrade" prop for heavy ballast.

However, there may be more going on here. Even under load, 20-30 MPH is in the usual operational range for either prop. It's just that a lower-pitch prop should get you there quicker and with less engine strain while accelerating (at the expense of a bit of top speed). Plus, you'll have the engine nearer its more efficient mid-range when surfing.

If your engine is putting out anywhere near its 300 HP, you should get up to speed. Derek, your original post said that it took a long time to get a rider out of the water with 10 people (and no ballast), and really talks only about planing... but what about the actual speed? Did it get up to the usual riding speed? If so, and you were not planing, replacing the prop (and possibly doing some engine maintenance) is not going to get you on plane, since planing at a given speed would be a function of the hull and the weight that it's carrying.

If your boat is just digging a trench in the lake... well isn't that the effect that we want for wakeboarding and wakesurfing? Yes, it takes more fuel, which is the price of creating a big wake. When explaining to newbies what a wakeboarding boat is, I often tell them that the point, unlike a runabout or a ski boat, is to NOT get fully on plane in the speed range for towing a rider.