If your 2650 would have the same "C" rating, then the 4000mAh lipo would give you almost twice the flight time. The "mAh" would be compared to the fuel tank in your car. More "mAh" means more miles that you can drive. The "C" rating is the discharge rate. The higher the "C" rating means the more power that the battery can dump to the motor. If you have a 20C lipo and give full throttle in a vertical climb, your plane would go "X" amount of feet with "X" amount of speed. If you have a 40C lipo and do the same full throttle climb, your plane would most likely go faster and higher.
Make sense?
You asked "How good is the 25-50C" I think that really depends on your application. I think the 25-50C range would be "average to above average"
When I take my Trex 500 out for casual flights (because I'm still learning and not ready for Smackdown 3D, lol), I use 30C lipos. Now...if Nick Maxwell or Bert Kammerer would to take my 500 out for some full-out "in your face, WTF was that 3D', they would probably use 70C+.
I hope this helps.