I'm certainly no expert at sound deadening, but I've tackled projects like this on previous cars - in a similar manner. I think the primary reason you didn't get the results you were looking for (nor did I on my previous cars) is that you really need to take a three-layer approach as outlined here:
Sound Deadener Showdown - Your Source for Sound Deadening Products and Information
The layers, from the bottom up, should be a 1) dampener, such as Dynamat or even Frost King from Lowes, 2) a closed-cell foam and 3) mass loaded vinyl on top.
Dampener materials don't have the necessary mass to knock out much sound, they only dampen vibrating panels (still important, though). Ideally, you'd want to use lead for its optimum sound reduction, but it's restrictively heavy and difficult to use...hence the mass loaded vinyl as a substitute.
Most of the big name products out there that people tend to use fall more under the 'dampener' category, and don't have the acoustic suppression properties people are looking for.
I'd recommend getting a digital sound level meter (Radio Shack has one for around $50) to measure sound levels on some test roads at set speeds to monitor results.
Just my two cents, though, is that I'm shocked at how refined the Z is. It measures about the same dB levels as my daily driver ('08 Mazda3) and really only exceeds it on horribly rough pavement. I was sure that I'd be looking into a project like this once I got my Z, but after a month of ownership I don't think I will - that shouldn't detract from what you're trying to do here, though. Good luck.
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