I can speak from some experience here, at least regarding the "Phase 1"
2.5L engine. I have one in my '96 Outback, and it developed the two most common headgasket leaks at about 110K miles. Well, actually the coolant leaking out problem developed first, on the left side, and I was able to manage that with some Stop Leak. The second HG blew in what I've found from my web research to be the more common failure, a rupture in a very narrow area between the cylinder bore and a water jacket race. Two things manifest themselves when that failure happens. First, hot gas is forced into the water jacket, causing coolant in the vicinity to vaporize and thus create a steam bubble. That steam eventually percolates out past the radiator cap into your overflow reservoir where it makes a characteristic gurgling sound. Some coolant can also be sucked into the cylinder thru that rupture, which causes a loss of coolant overall. Bottom line is, you think you have a coolant leak, you keep topping off the system, and the coolant keeps disappearing, eventually leading to engine overheating. No funny sounds, no radical change in cold cylinder compression (so you don't see it with the usual compression test), and nothing but the mysterious disappearing coolant to tell you that your Head Gasket's been blown. That's the symptom most reported by 2.5L owners.