How come many piston engines on aircraft have an output at 2700 RPM is it a special number or something?

Yes, in fact, it is. There is a definite limit that the sound barrier imposes on a propeller's efficiency. 2700 rpm represents a compromise between the tip of the propeller breaking the sound barrier (which lowers propeller efficiency) and optimum propeller efficiency. Typically, the longer a propeller, the more slowly it turns. Racing propellers are typically shorter, and turn at higher rpm: 3000 - 3300. But even with racing props, there is an ultimate efficiency attainable at high speeds.