I'm new at the Subaru specific stuff, however, I've had lots of experience with other types of performance cars, so here goes.
As far as the switchover stuff between regular and synthetic: When Synthetic oils were first introduced (eighties), most cars were still using paper and cork gaskets to seal oil lubricated areas. Over time, these gaskets would be saturated with conventional oil that "scabbed" or "coked" over due to heat cycling. Introducing synthetic oil to these surfaces removed the "scabs" and caused the leaks that were already there to become obvious. Synthetic oil doesnt "scab".
Most modern engines (I imagine the WRX is included in this, though I have not personally dug deep into mine yet) use rubber o-ring type gaskets or rubber-bonded metal gaskets, which are pretty impervious to changing from dino to synthetic. Switching back and forth should still be avoided if possible, because the two types of oil are incompatible, and it could leave you bearings unlubricated for a small amount of time. Changing over at higher mileage should not be a problem.
It should be noted that in general, conventional oil should be used to "break-in" an engine. There is a certain amount of wear that needs to occur in most engines and synthetic typically keeps this from happening properly. Failing to break in a motor properly can result in lower longevity, oil consumption, and a load of other bad stuff.
99% of all engine wear occurs at initial start up. Synthetic oils leave a film on your bearings even after the motor (and TURBO) haven't been running for a long time. Dinosaur oils do not, thusly synthetic oils protect your motor (and turbo) at it's most vulnerable time. You can tell the difference in the bearings of a motor that has been run for 100K miles on synthetic and conventional oil.
As stated before, Synthetic oils do not "scab" like conventional oils do. This offers a ton of extra protection for your turbo, because even if shut off hot, the oil won't "coke" up like a conventional oil. Remember all those turbo cars in the eighties? Domestic manufacturers had such bad experience with turbos, that most of them still won't build another turbo car. A lot has changed since then (water cooling is more common, better bearings and overall design), but conventional oils played a large part in the deaths of many many eighties turbos.
Synthetic oil does have the capability to last longer than conventional oil. In an enthusiast or racing application, it is not generally recommended to make use of this longer time frame for oil changes. Every time the car shuts off, gasoline, other pollutants and heat soak try to break down the oil. Even good synthetic breaks down.
Racing sythetic oils:
Royal Purple, Redline and other manufacturers offer racing types of oil in addition to their standard types of oil. These racing oils should never be used in a street car (though the standard stuff from Red Line, and Royal Purple is very good. The racing oils are extremely thin, and contain power adding combustibles. The power adding is good, but the lack of protection for the bearings isn't. These oils are designed for use in engines that are rebuilt every year or so. Even drag cars will typically only use these oils for one race weekend.
I plan to do the same thing with with my WRX that I have done with all of my hot-rod stuff. Afer a suitable break-in period (1000 miles), I am going to switch over to Mobil1 synthetic oil, and change it out every 3000-4000 miles.
That's all I can think of right now.