would motor 300 work fine with 8-10amp speed controller?
also what does motor 400 take?

Unless you set it up to draw more than 10 Amps, a 10 Amp speed controller will work fine with a 300 motor. How many Amps you draw is much more important than the physical size of the motor.

400 motor should have a 20 amps esc. Depends on the gear ratio, prop, and battery though.
I use a 30 amp with my brushless 400.

But, then the more amps it puts out.. that means the battery goes dead faster right? I wonder how many amps the generic esc puts out in my Thawk. Some ppl said to replace it, and others said its fine. It seems fine to me, but I never tried other esc's in planes. I had a mechanical speed control in my race car and it was horrible, then I got a esc, and it was MUCH better.

The esc you have is fine if it works okay.
If you add more cells to the battery pack or use a bigger prop, then it might not be big enough anymore since the amp draw will increase and, as you say, the battery will run down faster, and the esc might melt if you esceed its 10 amp rating.
The higher the amp draw, the larger capacity batteries you need so your run time will stay the same. Your esc is also rated for a maximum number of cells.
If you change anything it's important to know the amp draw so you don't overload anything and cook it.
Motors, esc's, and batteries have a maximum amp draw.
The best way to know is to get an amp meter. I like the AstroFlight Whattmeter. It connects inline between the battery and esc. As you run the motor it shows the amp draw, and the battery voltge and watts on a digital display. It's a great tool to optimize performance.

The motor/GB/prop combo determine how many amps are needed for a given number of cells. The battery size must be capable of delivering the amps and the ESC must be capable of "processing" the amps.

Sierra Gold

Yep. The ESC doesn't "put out Amps." The motor "sucks Amps" and the ESC only passes them straight through.

Electricity is what I term a demand-based energy source. To get more power, you increase the load. This is contrary to say, steam, which is a supply-based energy source. To get more power, you increase the pressure at the boiler.