Here are your values with values added for what I actually observe (question marks indicate fuzzy memory. I usually don't cruise at those speeds and never made a chart so might not be accurate). Engine is rated for 4k RPM so I usually cruise between 2700 and 3100, only occasionally bumping it to 3300 when I want to make time.

Speed - rpms - gph - Actual observed
5 ----- 1660 -- 0.4 --- 5?
6 ----- 1960 -- 0.65 -- 6?
7 ----- 2120 -- 0.75 -- 6.5?
8 ----- 2420 -- 1.15 -- 7.1?
9 ----- 2720 -- 1.6 --- 7.5
10 ---- 3020 -- 2.2 --- 8.1
--------3300 ---------- 8.5
--------3500 ---------- 9.2?
--------3900 ---------- 9.7+?

As for whether the boat is overpowered? Maybe, but there are also factory options for a 105 and a 125 HP engine. Each of those cruise above 10 knots. A friend who owned the 150 cruised at 12 knots with a top speed of 15 under power (bow up) and yes he had cavitation damage to his prop blades. (It actually wore holes in the bronze blades.)

The strongest steady headwind we have been in was 45 gusting to 50 knots apparent, and I was glad to have the power. We maintained about 7.5-8 knots boat speed, and that got us out of the nasties a bit quicker.