Seems like you are in exactly the same boat as me. I also have an 06 xt and have spent a lot of money fixing suspension, brakes, bushes. I've come to the conclusion that I'm also so far in it the only thing left is to push on. Although I did buy the Subaru as a cheaper, more usable car than my Audi RS6. If anything it's been more expensive!
I've accumulated parts for a service including plugs which I was hoping would solve my misfire/stuttering, but seeing as you have now replaced everything and still have the issue has left me slightly concerned that it's going to be another expensive job.
And to make matters worse, I went out in it last night and noticed I now only have one headlight bulb working. So either the ballast is knackered or the bulb has blown.
i will be keeping an eye on this thread with interest. If I find a fix to my misfire I will let you know!