Great initial review. I have also just purchased the TrackIR and am slowly getting to grips with it. The increase in immersion is far greater than I was anticipating, and after a few hours it started to become natural, looking around the car and into corners. As said in the SRT review, with the TrackIR the helmet cam in Race07 finally makes a lot of sense!
I gave LFS ago without the headtracker after using it for a bit and it felt like my head was in a vice! However, I need to tweak the settings a bit before it is perfect. When I get a profile I like I'll link to it here for you to give it a go.
I had been thinking about saving for a triplehead2go setup for a while now, but tbh I think that by upgrading to a slightly bigger monitor and using the trackir I can get a similar effect, thus saving money and desk space.
Do I think the trackir makes me lap faster? In a word, no. It certainly helps in spotting apexes and comes in especially useful for situational awareness in tight racing, but I don't think it will cut your laptimes. In fact, it took a while before I felt 'connected' to the car again, as until you have 'trained your head' it can be hard to see the visual cues as to what the car is doing. I have not raced online with it yet, but I can see my racecraft improving as I get to grips with it more. The main advantage is that of the immersion and illusion of reality that headtracking provides, which after all is the reason we simrace at all, isn't it?
Let me know how you're getting on...