I've installed the spacer on saturday, the install was very easy even tough the instructions could use some work. Also I had a leak after installing the spacer since bolt #2 has bottomed out, I had to get a metal washer to raise the bolt. Also I've stripped the thread in the lower manifold with bolt #7. The supplied M6 x 30mm bolt was to short and I had to get a M6 x 40mm bolts for the front. Also fitting the strutbar was quiet a headache, since I had to play around with it to clear the plenum and the hood. I ended up using 3 washers per bolt on the drivers side and 2 washers per bolt on the passenger side. The great thing is that it looks totally stock unless you know what you are looking for since you get to keep the OEM strut bar and engine cover.
As for my impression, the first thing I've noticed that the idle was a lot smoother. In my car at least, when the car would idle it would try to stall, I could feel two knocks coming through to the cabin at that time, then it would recover, I think . Now the idle is a lot smoother, it's not perfect, but it's a lot better. Maybe grounding kit might make it perfectly smooth.
As for performance, I can't feel any gain down low like some people have reported. But at 5,000 rpm when usually the engine started to feel a bit flat, it starts pulling quit hard all the way to the redline, I was quiet surprised. I can really feel the difference around 6,000rpm, also I've bounced it off the rev limiter first few times, since it was accelerating faster then I'm used to. Also I'm not getting any detonation like Zuff has experienced.
As for sound, it doesn't make any, but the popcharger is slightly louder and I can start hearing it a lot sooner then before. Also according to Crawford popcharger works a lot better with their plenum then any other intake and they have seen the best power gains with it.
In us people have been getting around 10-15rwhp from this mod. That's around 7-11kw at the rear wheels. I would say that it's the best value for money mod, definitely beats all the exhausts in the value for money.
There are other options as well, right now there's a motordyne spacer group buy on my350z.com, which has similar performance gains but it's not angled like the one from AAM. Also Peter P. from Sports Cars International has one some place in his workshop (and he can make some more for a lot cheaper then I got mine for), it's similar to the motordyne one, it's 8mm thick from what i recall vs 6mm from motordyne (they also have thicker ones, but they don't fit under the strut bar).
Overall I'm very happy with the upgrade, I wouldn't mind doing some rolling runs against another stock and hi-tech (or aps) equipped 350z Track to get a better idea of the performance.