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I finally had a chance to try out the new Spitefire 13.8" x 11"p prop. It made a BIG difference! It was like having a shot of nitrous for the start.
I have to throw a wrench in this information because I also added a SE Sport 400 no-drill Hydrofoil.
With two people on board and two people in the tube, about 625 pounds of people total plus 100 pounds plus of stuff, we were able to get on plane in 5-7 seconds. Max RPM's loaded 5900-6000. The bow never came up high enough that I could not see sitting down. Before I was NOT able to get on plane with four people, two in the boat and two in the tube, it had to be two and one. Now it acts like we could add two more people. Top speed with or without the tube in the water was 24-26 mph, with the old prop it was 32-35mph depending on which side of the lake we were on.
Without the tube in the water the engine reaches 6,000 rpm at 3/4 throttle, and you don't get any more speed even with one person, 26mph was it. The downside is you have to be careful not to exceed 6000 with a light load.
This will not be my every day prop because I lose so much top end. I will use it when we are tubing, because it is great for tubing. Compared to a friends 18' bow rider with a 115 FS we were able to out run them slightly pulling tubes. He had about the same people and gear load, but his boat weighs double what my Whaler weighs.

***The only issue I had was at idle speed there was a notable vibration from the engine, but as soon as you picked up a little speed it went away. My theory is the prop wash is cavitating against the hydrofoil, but that is just a theory. I will try it without the hydrofoil when I get a chance.