Crist,
It is a good question that I don't completely have an answer for. First the watt rating of a motor is sort of a "burn-up" quantity. It certainly isn't where that motor will run nice and efficiently.

When I was going for my Nobler (after the Super Clown), I had a rough idea about what I needed to make the Nobler fly (rpm and prop size). I knew that the AXI2826 seemed to be a defacto standard for CL stunt (based on what other guys were using), so I guessed that was nominally what I needed. I began to look at RCGroups Electric forums, and about that  time Scorpion had just come out with their kit motors. Also they provided tables of the motors with props running statically with currents, watts, voltages and rpm. So I just crossed correlated that I needed about 10krpm on a 10-6 prop to fly the Nobler, and that the total power package weight shouldn't be a lot more than my Brodak 40 with a tongue muffler and a 4oz (filled) fuel tank.

So I looked at the Scorpion 3014, 3020, and 3026 motors (sort of equivalent to AXI 2814, 2820, 2826). When  I looked at the weight difference between the 3020 and 3026, and how they handled power, I decided that the 3020 made sense. It turned an 11-5.5 prop just great (what my mind had gravitated to after flying the Super Clown with a 10-6) .I also bought a 3014 kit at the same time. One thing to remember, it is easier to get a low kV with the bigger motor (fewer turns). In the Scorpion line, the 3014, 3020, 3026 weights are 4.3, 5.5, and 6.7oz respectively. The Max wattage ratings are 600, 800, 1000 respectively.

I have also flown the Nobler with the 3014 and it flies just fine with that too. If I was going to buy a new motor for the Nobler, I am thinking I might just go with the 3014.  Recall that my level flying wattages (input) are in the 250-275 watt range, and only briefly peak into the 450 watt range. Since the wattage rating as I mentioned are continuous burn-up type ratings, I don't think I am stressing the 3014 at all. Now whether the 3008 would work well or not I am not sure. At some point you do need to balance the plane! (also the 3008 is still less efficient than the 3014, and at some other point, you need to make up for inefficiency with bigger (=more weight) batteries. So for me, and my choice of 18C 2100 cells, the 3014 and 3020 will certainly keep me happy for planes in the glow 25-40 range.

Crist
Here are some" rules of thumb"that will help you with your choices and put you in the ballpark. First decide in your own mind if 400 watts is at 100%throttle or 80% .What ever you decide stick with it in all your calculations.
 Motor weight :To get usable power multiply motor weight in GRAMS x 3 ,to get maximum power multiply by 3.5.Example 4.0 oz X 28.35 = 113.4 grams x3= 340watts
usable power,and 396.9 watts maximum power.
Find out the constant amps for the motor published by manufacturer.
Chose your battery voltage.3 or 4 or 5 cell or A123.
Plan on your Kv range by using these 2 formulas:
  Kv x nominal volts x .7 = Target rpm ( Use 10.7 for 3cell,14 for 4cell,18 for 5 cell.)
  Pitch speed(51mph) = RPM x prop pitch Divided by 1056.(or Use your own P. S.)
Look for the motor that has the lowest Ri, internal resistance even if it slightly heavier.
After you have selected your motor choice(s) you should try to run some calculations
on motocalc or other calc programs like NEU which is now on the CC website. To verify that you wont pull too many amps. Remember that most programs are run for 100% throttle.You will need to adjust for your throttle plans.
 Good luck,
 Walt