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Dave, I think you set a new record for the longest meaningful thread time gap!

Your motor is mounted way too low. Your lower unit is dragging in the water at all speeds, causing a loss of efficiency and potentially hard steering. You may also be noticing some porpoising when trimmed out for speed, and excessive splashing - those are also symptoms of being way too low.

First raise the motor two holes - you may be able to go up one more hole (to all the way up) but two holes up will work with almost any prop. Then go give it a test run, loaded fairly light. Based on your previous reports of performance, it is likely you are going to find you're able to go above the maximum recommended RPM (6000). Your motor has a rev limiter, so you won't hurt it, but I still wouldn't run it more than about 6100).

So now you will very likely need a new prop. You state that you want to stay with aluminum, yet you are interested in efficiency. Stainless steel props are certainly more efficient and perform better in general. Some people want to stay with aluminum because they have heard the old wives' tale that an aluminum prop will save your motor's gearcase in the even of a rock strike. That's simply not true - all props today, both stainless and aluminum, are made with breakaway hubs that work to prevent damage. If you hit a rock, you may or may not damage your gearcase with either type of prop.

So my recommendation would be to go with a good quality, stainless steel prop. You will likely want to go up in pitch, but be aware that pitch is only one factor - blade size and shape matter as well. In general, for 4-strokes such as yours, a prop with less blade area lets them spin up more easily into the range where they make their power. You'd certainly want to stay with 3 blades, not 4, for the same reason.

You may need to experiment a little to find the correct pitch, so if you can find a way to try before you buy, that would be perfect. Some prop manufacturers such as PowerTech offer both advice on prop selection, and a trade-in program to let you swap for a different pitch if needed. I went with PowerTech, purchased online from Dan's Discount Props, and was very pleased with the results.

Good luck with it!

Tim