I performed this procedure last evening. Start time was 5:00 PM and I was able to finish the test drive at 10:30 PM. The dash removal and reinstall was relatively straight forward. This is more time consuming and tedious than it is difficult. Interior trim removal tools were a life saver.
Chris is right. A very likely culprit to the rattles are the dash plugs and their firewall sockets. What I did was remove all of the sockets and put a VERY healthy bead of RTV behind them where they mate with the firewall. Foam padding was also placed inside the sockets just as Chris suggested. I then wrapped the plugs with electrical tape and placed a bead of RTV on them.
This removed almost all of the "plasticky" rattles. However, there are some metallic rattles that this procedure did not remove and I will be digging into the dash once again this weekend to hunt them all down. I will also perform the fix per the TSB for good measure.
I would like to comment on how Subaru knew that this was a potential area for rattles. The sockets have a very fine sticky-backed foam behind them. Unfortunately, this foam backing had become brittle over time and nearly flaked off when they were removed.