I have a Turnigy 4250-580kv motor that is rated at 4 to 5S according to the HobbyKing website.  I want to run a 6S pack and keep the rpm around 10K.  What do you think?

Motor:
http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=8506

Battery:
http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=9462

BTW, the Pathfinder I was given at the FCM contest is ready to go with the above and a CC ICE 50 Lite.  56 ounces!  620 square inch ARF electric at 56 ounces.  It'll be a good one....I hope!   

Hi Crist,
 It has been my understanding that as long as we don't exceed the maximum rated watts of a motor we can vary the cell counts up and down.(That is as long as we have a speed control that can handle the voltage.) I ran that setup in the Castle link demo mode and it told me that you are good from 8200 RPM to 11000 RPM on a 6 cell battery. It looks like you will be fine.

There's some funky electromagnetic effects that can cause the iron in the armature to overheat with too much voltage.  But long before that happens you'll spin the motor so fast you'll damage the bearings or tear it apart with centrifugal force or otherwise do it violence from purely mechanical forces.  With some speed controllers, before that happens the speed controller won't be able to keep up, and it'll be a moot point anyway.

I suspect the "cell rating" of the motor has more to do with trying to get you into a range where a normal sport plane with a normal prop will be happy, although it could be Turnigy's way of telling you how fast the motor can spin before it smokes the bearings or bends the bell or whatever.

You will do best if the full throttle rpm is around 75% of the rpm calculated from the Kv times the battery voltage.
Almost all or out outrunners are most efficient if the FULL THROTTLE rpm is in the range of 70% to 85% of the Kv calculated rpm.

With this rpm requirement met you will get longer flight time for a given battery size or can use a smaller (lighter) battery for a fixed flight time.

Experimentally Determing if this condition is met is difficult experimentally but a calculator like moto-calc should work.

That has to be the cleanest installation I have ever seen on a profile.  I studied that first pix for a long time before I figured out how you mounted the motor.  The looks like a really secure battery strap too.

FORGOT!  Back to your first post:  On my Pathfinder I have an Arrowind 2820 turning a 12x4.5P (clipped 13") turning 9700 RPM on .015x63' This dovetails nicely to your 10K target - you ought to have plenty of headroom and a very efficient set-up.

When you refer to a pathfinder thats a big pa model and the smaller one  1/2 one is a half -A pathfinder?

The full size profile of 620 square inch wing area.

Crist,
When you cut the nose off, be sure and get all the oil and fuel residue cleaned up real good! 
Will

Ha, Ha!  You kill me man!

My goodness, I hope not!!!

There are 2 main things determining number of cells for our BLDC motors:

1/ maximal allowed RPM which is usually somewhere at 15000 on our full stunter motors (smaller will be higher). This does not limit us as Will already wrote, because we use governor and it will not allow high RPM even without prop. But be carefull if you calibrate ESC, some ESCs try to run motor without prop to its max RPM.

2/ However point 1/ is not valid on LOADED motor and that is real limitation, if you do the math, then you will see that on FULL THROTTLE and loaded motor under its max RPM limit you are going to exceed not only current of good efficiency, but also highest allowed current. Simply too high voltage will not allow RPM in safe range and also save current at the same time.

So those 2 things determine rated cell count. Howeverer we need some head room for voltage, especially with active timers and our ESC is never open for full 100%, so usualy we use max num of cells +1. If you go higher, you will face troubles mentioned by Tim, especially ESC overheating.