My thought has often been that with a turret up front and at the rear and a single beam up front and at the rear you could, potentially, always have at least 3 weapons on target along with lower power drain when firing all 4. At least that's been my hope.
I ran a couple of quick tests and I verified that you can determine the DPS of different weapon combinations by averaging the different results. So, for example, if you want to figure the DPS of 2 turrets and 2 beams, take the damage of 4 beams and 4 turrets, add them together and divide by two. The only thing to remember is that you need to take the figure for the total number of weapons being equipped, rather than the number of each (ex: average 4 turrets with 4 beams, not 2 turrets with 2 beams).
This average gets you in the right ball park, only +/- a few percent, which could very well be due to the small sample size of my quick verification tests. I even tried a very weird build of 1 turret, 1 beam, 1 cannon and 1 dual beam, and the calculated result was about 4% lower than the actual result (once again based only on a single test run). While this may not be 100% exact, it should provide results that are close enough to judge the effectiveness of a build.
Unfortunately since i haven't done any low power tests, it's hard to say right now how turrets vs beams would look at a lower power setting.