I've been editing them in-game. That way I don't mistakingly put a wrong value in! The game has the values listed as none, light, normal, heavy, very heavy. Which means in the text file, that's 0, 0.5, 1.0, 1.5, 2.0 respectively.
I haven't found the designs to be better exactly, but it does give you more control to make each race unique, though why missiles and rail guns aren't included is unknown.
I've found that giving a heavy value in beam weapons will result in the smaller ships having less or sometimes no torpedoes, even if you've set the torpedoes to heavy also. So you can see that it's balancing everything in terms of energy usage and ships speed.
I haven't done alot of testing, this is about all I really know.
It might be worth setting up a race with low values and a race with max values and pit a design of each against each other. Probably best to make it a high and equal tech level battle between 2 ships. That would be interesting. I would expect the ship with max values would win, but maybe not as easy as we think...
Sorry I'm waffling, maybe I should go and play the game!!