First, it's an i7, it's hyper-threaded, two virtual cores. That's one core in your shot, not two. The game is loading everything on the #4 core. This is part of your performance problem, perhaps. Sins is one of those games where hyper-threading is capable of reducing performance.
Unfortunately, the symptoms you describe are that of a pegged CPU, 100% utilization of a core, not 60-70%.
Assuming you have no such issues with the vanilla game, I'd suggest looking into the health of your install. E4X isn't particularly resource intensive, it shouldn't run that much worse than vanilla does. A broken file could be causing all kinds of performance hell though, data corruption could result in your symptoms.
If you do have similar issues, just less extreme, it could be a rather diverse array of issues, from a buggered directx install to a corrupted drive. It's also possible you have a contributing factor resulting in your hardware inadequacy, such as an anti-virus going haywire over the game files, or an encrypted file environment.
Tried running in window mode?
