Given these idle issues never seem to get resolved, I thought I should post the outcome.

As mentioned previously, I went ahead and pulled the crank pulley and checked the timing. The timing looked OK, but not perfect. It seemed like it was close, but possibly out 1/2 a tooth if I squinted hard enough. On that basis I ended up pulling the belt and setting the timing so that the mark on the pulley (passenger side / or US drivers!) was before the mark on the cover, not slightly after. This made the timing look not as correct as it is meant to, but it made the car run like a dream. The idle is now perfect. No misses or hesitations, it is running better than it ever has. On to boost, off boost, it just purrs and is rather sprightly. Red lining the puppy and it is alive! It is so quiet at idle, although the Dayco belt makes a lot more noise than the OEM belt. With this change, there is no miss like there was before I did the belt.

So the moral to the story is, if you do a belt, and it idles poorly, then you have to pull the covers and play with it. Retarding it ever so slightly may well fix the issue for you.. I spent about 8 hours fiddling (after initially changing the belt) to get it perfect.. I would hate to do that for my day job!

Good news is that no cam or crank seal leaks (replaced them all) and the water pump and thermostat are all flawless. Now I can go back to enjoying my WRX'y