A discussion on Watts/pound that I offer just for general reading, not directed to anyone in particular. Comments are welcome.
Note you can get high watts in two ways. You can go for a larger/wider prop or you can go for a deeper pitched prop. Both will drive up the watts/pound. However they favor different factors of flight performance.
A larger/wider prop tends to favor thrust which is what we look for on a thermal glider. Thermal gliders are optimized around lift at lower air speeds so we are not trying to make them fast, we are trying to get them to altitude quickly via a steeper climb capability. So we tend to go toward the wider prop that will produce the watts/pound we want. Think of this as working in a lower gear on your car in order to pull a heavy load.
Hot liners, pylon racers, etc. are built for speed. Here you spend your watts/pound on a narrower/deeper pitched prop that will produce a higher pitch speed. It might produce fewer ounces of thrust but will get the plane going faster in level flight. Think of this as high gear in your car to produce more speed but it might bog down the engine if you were trying to pull a heavy load.
The difference is that cars can go through a series of gears to get the power type desired at the speed desired. Aircraft with fixed props can't do that so we pick the compromise that works best for what we are trying to accomplish.
So, watts/pound defines a power pool that we tap into to achieve the type of performance we want. When I look at motor specs I am typically looking at what they will produce with the wider props. So lower Kv rated motors that spin big props but perhaps more slowly.
These are not hard and fast rules, they are generalizations. That is one of the reasons I turn to the calculators when I am trying to match up props with motors. Or I buy several different props, put them on the wattmeter to be sure they don't overload the motor, then run tests in the air to see what gives me the kind of performance I want.
What performance are YOU looking for?
The concept of over powered or under powered has to be measured against some performance criteria. Your criteria and mine may be different. 75 watts per pound is more than adequate for sport flying where time to altitude is more about convenience than any special target time or altitude.
I measure performance in how long it takes the glider to reach 200 meters. As an ALES contest pilot that is the spec I am most interested in when I am setting up a motor/battery/prop combination for a given glider. I have a device in the glider that cuts the motor at 200 meters so I time the climb at various angles to see what I get. My power target is typically 110 to 130 watts/pound. Higher works but this is my target range.
For example, my Radian, with stock prop, 9.75X7.75 reaches about 170 meters in 30 seconds at about 16 amps peak. Doesn't get me to the height I want in the time allotted. Change to a 10X8 prop and it gets to 200 meters in 28 seconds at about 19 amps. If I go to an 11X8 it gets to 200 meters in 22 seconds at 23 amps peak. I could try a 12X6 or a 13X? and see what they do if I wanted.
In doing the testing I take into account not only the time to height but the amps being drawn by the motor. Parkzone does not publish the max amps/watts for the Radian motor but I have looked at other "480" class out runners and they all seem to be able to take at least 25 amps on a burst so I figure 23 amps dropping off to about 21 is about as high as I want to push it and I don't need to get to 200 meters any faster than 22 seconds with a Radian.
I am only running for 30 seconds and that drops off to about 21 amps after a few seconds so I think I am OK. The motor will have 5 to 10 minutes to cool before I restart or land.
There is also structure to consider. The Radian is a foam glider. I am spinning an 11" prop now. The wider I go the greater the torque on the nose of the glider. If I put any bigger prop on this the torque of the prop might eventually twist the foam nose and warp or break it. I did see one person replace the Radian Pro motor with a very powerful motor and a 14" prop. Twisted the nose right off the glider.
My e-Supra is 68 ounces (3.4 meter wing) and the motor is putting out about 480 watts for 112 watts/pound. The motor/gearbox combo is spinning a 17X11 prop. The glider makes 200 meters in about 24 seconds and that is fine for my primary ALES contest glider. The Radian is my back-up.
So, just a discussion of considerations when you plan your motor/battery/prop combos.
Naturally, your smileage may vary.