Advanced timing can increase power, but going too far can also reduce it. Think of a graph that shows cylinder pressure (Y-axis) versus crankshaft degrees (X-axis). As the piston comes up on compression, cylinder pressure rises. At the point the spark occurs, the cylinder pressure builds very rapidly. As the piston passes TDC and begins moving down the cylinder, the increased pressure forces it down, doing work on the piston.
The goal of engine designers/tuners is to find the best compromise between maximizing the pressure in the cylinder at the moment the piston passes TDC while minimizing the cylinder pressure before that point. By maximizing the pressure at and after TDC, the amount of work done on the piston is maximized and by reducing the amount of pressure on the piston as it still moves up on comrpression, the amount of "negative" work done on the piston (i.e. the piston working against increasing cylinder pressure as it rises on compression) is minimized.
The mixture takes a finite time to burn, requiring the spark "lead" the piston by a given # of degrees of crank rotation, so that by the time the piston reaches TDC, the burn is nearly complete and cylinder pressure is at a maximum. Finding the ideal # of degrees to lead ("advance") the timing varies according to engine temperature, RPM, load etc. The PCM looks at these and other factors and uses a lookup table to determine the spark timing. This table (and others associated with it) were calibrated my Mazda/Nippon Denso to produce good driveability, reduced emissions, ability to run crappy gas, return good mileage (esp. at part throttle) etc.
The advance settings to produce best torque don't necessarily produce the best mileage, emissions, octane sensitivity etc. A PROM can be "optimized" to put the emphasis on power and not the other issues, though the power gains will not be huge.
You can go too far of course: advancing the timing will increase the area under the pressure vs. degrees curve before TDC, increasing pumping losses and reducing output.
For the V6s, a spark advance can only really be accomplished by a PCM upgrade since the PCM is in control of spark timing regardless of the distributor system. I haven't worked with the ATX 4-cyl EEC system enough to know if rotating the distributor si enough to advance the timing (5.0L Mustangs using the EEC-IV system allow this I believe...)
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