Okay, here we go.
Pickups are a hunk of metal with wire wrapped around it. If you add more wire, you'll get a higher output. If the iron content of the metal is higher, again you get higher output.
When the output goes up, The highs and lows increase at a greater rate than the mids do. This produces the "balanced sound" of those expencive pickups. You can do the same thing if you turn up your amp or tweek your EQ. It's sometimes called "scooped mids."
Sence a guiter lives in the middle of the sonic spectrum, adding these extra highs and lows only serves to muddy up the overall mix.
When a guitar player sounds good it's because he (or she) has found his place in the sonic spectrum. They don't have to be loud because their not fighting with the bass or keyboards or whatever for space in the music.
If you play solo all the time, or your band doesn't have a bass player, then higher output pickups might be worth the extra effort. But, if you play in a band your better off with your stock low output pickups with the more mid range tone.