The game is 720p, not 1080 on the Xbox 360.
Anyways, stop blaming performance on ports. Very rarely are games programmed with only a single platform in mind. Also, the increased complexity of software and need for cross-platform development necessitates the need for APIs and makes platform-specific optimizations unfavorably.
Just look at Ur Quan Masters, the remake of Star Control II that's designed to run on multiple platforms. The original ran on a 386. The new version requires a 200 Mhz processor. The new version is multi-threaded, using ogg for music and png for graphics (I think).
So it's not the fact that it's a port that matters, it's the fact that most games are designed to be portable that affects performance. The Xenos has a very efficient unified architecture that is superior to any x1k or 7-series card. That's also why this game runs so much better on the 8800. Basically low settings is what "optimize for the typical, aka non-unified shader PC" is. Now if it still runs poorly at low settings, then you can argue that it's a bad port/optimization/whatever.