The game feels like a very different beast now that I had a chance to upgrade my main desktop to an 8-cores Mac Pro.
After installing bootcamp and migrating my files (including my customizations of Morrowind and Oblivion), I could take the game for a drive on this new machine. Multiple cores really make a difference compared to Hyperthreading, even though only two out of the 8 cores are used at any given time. Spreading the load of .bsa files across two SATA drives help a lot as well.
For now, I am going to leave most of my optimizations in place (except for re-enabling the flickering of candle light). With these optimizations and the new hardware, I am finally reaching framerates between 30 and 70 fps in some outdoors areas while remaining above 20 fps in some crowded city areas.
Still, even with the increased power, the game tends to crash a lot more if I try to play with a resolution above 1280×900 pixels. And since I had to trade my ATI card by an Nvidia, I have been confronted to the ugly Nvidia black screen bug.
The fun of troubleshooting just never ends…