Basically, the small gap makes a plug last longer, yes. The larger the gap, the more voltage that is required to spark across it. Higher voltage means faster plug wear. 0.040 is a good comprimise between "runs good" and "last long" As far as your 85 Ford is concerned, it's like comparing apples to oranges. The Subaru ignition system is not all that much better, but the overall efficiency of the engine is. A really efficient combustion chamber design with good fuel management really negates the need for a super hot spark or multiple spark ignition systems. Also keep in mind that as a coil ages, it weakens, especially the older oil-filled coil like on your Ford. How old was the old coil? It's quite possible that you would have noticed similar gains from a new stock coil.
Keep in mind that no matter how much voltage a coil is advertised to produce (some MSD coils claim to be 150,000 volts) the actual firing voltage is only the amount required to make a spark. In other words, if the spark plug only requires 15,000 volts to spark, that's all the coil will produce, regardless of what it is capable of. The spark voltage that is required varies widely during normal driving, anywhere from 7k to 25k volts.