Well, it can get complicated. I covered it before in my VTX guide:
https://oscarliang.com/choose-video-tran...ower-range The more power you have, the "louder" the VTX is. So longer the range.
However, most VTX antennas transmit in a sphere (or doughnut shape). You have to distribute this energy over the surface of the sphere. So say you have a certain amount of mW/area. The further away you move from the center of the antenna, the larger the area of the sphere, the less mW/area, so the signal is weaker. You're range is going to be like the radius of this sphere.
Because of the properties of the sphere, doubling the radius will not result in a doubling of the area. In fact, a tiny increase in the radus, can result in a massive amount more area of the sphere.
Likewise, doubling the power, and then going to a distance where you have the same mW/area, will not mean you have doubled youre range.
So you cannot go from a 400mW, to a 800mW, and expect to double the range. More like 1.2-1.5 times the range.
Negatives of high mw VTX:
- draw more current and generate more heat. So overheating may be an issue
- high mw VTX generally are more expensive
- high mw VTX may not be legal, most places in the world only allow 25mW
- It makes it harder for others to fly with you
Generally, the maximum you will want to go is 200mW when flying with others. but it's nice if you can get everyone on 25mW.
If flying alone, 600mW is fine if it's legal where you are flying.
As for range, I found antenna plays a much much bigger role
Here is my antenna guide in case you're interested:
https://oscarliang.com/best-fpv-antenna/