Mixing myths in missives means mistakes made.

If motor A and motor B have the same Kv, then the props and batteries remain the same.

The "mythical 80% goal" that I believe you are targeting is the amount of battery capacity used in a flight -- not the % use of the motor's power rating.

Double-check your power ratings.  The recommendation that I've seen for motor power is 7W/ounce continuous, 11W/ounce peak.  That works out to 110W/pound and 180W/pound, respectively.  If you've got a motor that lists separate continuous and peak power ratings, then go with that.

The motor power rating doesn't have a huge amount to do with how much power you can get out of a motor.  It has to do with how much power you can get out of a motor without melting it.  If your plane is flying right and the motor isn't too hot to touch at the end of a flight, your motor is OK.

If you're not getting enough power out of a given motor/prop/battery combination, then you need to change something to get more power, not to handle more power without burning up.  Assuming that you don't just need to increase your launch RPM, you need another cell on the battery, or a higher pitched prop, or a motor with a higher Kv.  (Note that both the higher voltage battery and the higher Kv motor just enable you to increase RPM -- you still need to do it).

If your plane is inadequate with a 500W-rated motor of a given Kv, and you replace it with a 600W rated motor of the same Kv without changing anything else, you'll make the plane heavier and move the CG forward, but you won't change much else.

Thanks for the response, Tim.  Your last statement really clarified the crux of my confusion.  Yes, both motors I had used/considered had basically the same Kv numbers.  I felt that I wasn't achieving anything, but was hung up on the idea that I was considering a more powerful motor, so why not?  Got it, now.  I'll go back to my prop selection choices which may require a similar motor with a different Kv to keep from burning up the current motor.

You're still a bit confused.  When you start overheating a motor by changing props, then you need to consider a higher power motor.  You can get fancy and go to a higher RPM setup (either more battery or more Kv) along with a flatter prop, but if you're happy with how the setup works except for the unhappy motor, and if you can stand the extra weight, just go to the bigger motor.

Hello again Jim
Thanks for the PM. Let me try to answer your question ...
First, some general info about the plane you suggest.
1) It takes about 0.6 to 0.7 Watt-hours (that's a unit of energy not power) per ounce of all-up weight to fly the Stunt Pattern. That encompasses the entire 6-minute flight, and assumes the timer has not started the motor until after you have picked up the handle. That 47 ouncer will probably consume just a tiny bit less than 2.0 Amp-hours of capacity from a 4S battery. The average current for the flight will be 20 Amps, and the peak currents will be maybe 1.5 times that, or 30 Amps. That's 275 Watts or so average with 425-ish peaks.
2) assuming you use a 900 RPM/Volt motor (KV=900) and you "cruise" at 75% throttle under the governor's control, that works out to 9700~9800 RPM. Period ... That's the RPM you'll be running, give or take 15%. This is dictated by the motors current vs RPM line at 75% of 14.4V. You could set up for RPM corresponding to as much as 85% of the battery, but things might get hairy in the clover on a cold day. (Batteries deliver a little less energy on cold days)
Now, you could end up running a large diameter prop with less slip and a LITTLE less pitch or you could turn a smaller diameter prop (smaller disc area means more slippage) and a little more pitch.
The bigger diameter prop will pull a little bit better uphill, and brake downhill better, its keeps the airspeed a little bit more constant.
On the other hand, large diameter may kill the corner and cause problems with level flight groove if the prop is too long and or too heavy.

Also, there are places in the schedule where the momentum of the plane tends to keep apparent ground-speed constant (think the outside edges of the overhead despite having just turned into a headwind. This also argues against very large diameter.

In short, you will end up playing around a bit, but don't expect large pitch changes to accompany the diameter changes. Oh yes ... and remember the pitch number stamped on the prop is only a little bit better than a fairy tale.

I hope I helped,
  Dean P.