Quite literally, the only difference within a given series is the gear ratios. A higher gear ratio allows you to spin a larger propeller at a lower RPM on the same amount of power. You're gaining thrust at the expense of speed. More acceleration, less top-end. The key is in finding the right mix for the specific plane.

Talking specifically about GWS stuff, you can usually replace an EPS300C with an EPS350C of the same gear ratio for a performance increase. What happens is the EPS350C's "hotter" motor spins faster, in turn spinning the propeller faster. Nothing is free, though. The EPS350C will draw more Amps to turn the propeller faster. This means better performance at the cost of shorter flight times.

An EPS400C isn't really comparable. It's a much larger, heavier motor that requires a completely different power system. Apples and oranges, you know.

As long as the ESC you have is rated to handle at least as many Amps as the power system will be drawing, it's not an issue. Doesn't matter who you buy it from.

You can see charts that show the relationship between voltage, current draw, and thrust at http://www.gws.com.tw.