Thanks for that interesting report!
I recently encoded a 30 minute HDV-project (from Premiere Pro 2) into WMV-HD. It took my 5-year-old 2.4 GHz PC a little more than 24 hours.
Settings were average 9.000 kb/s in 1080p and 2-Pass.
It took your Mac Pro about 16 hours for 2 hours of footage into h.264, right?
That would mean that the Mac Pro is approximately 6 times as fast as my old PC assuming that the encoding settings are comparable.
I thought it would be even more but it could be that h.264 is more complicated to encode.
Did Premiere use all 8 cores?
MPEG2 must have been a lot faster. I understand why you would have preferred h.264 though. You could fit more footage on one disc at similar quality. But if you use a bitrate that´s high enough, MPEG2 can look very good.
By the way: If you are encoding your HD-footage just for private use, you can also just use WMV (if you have the codec) since the PS3 supports that since the latest firmware-update. You can then burn that file to a DVD, or even a CD (if its small enough

) and watch it on your PS3. HDV-footage also works but you can´t burn more than 20 minutes of HDV onto a DVD.
By the way, do you by any chance know if there is a way to export an h.264-file that the PS3 can read? It should support that codec too.