Crappy $30 controller card was limiting raid performance. Card is unable to set drives in AHCI mode, which proved to be an issue.
So what did I turn to? Windows 7 software raid! From all reviews I've read, windows software raid is actually quite fast, especially when paired with a fast and capable CPU. I tried it out and everything is running smoothly. Now getting ~480mb/sec and ~180mb/sec seq. read/write, couldn't be happier with this!
BTW, thanks for the input PeterUK. You are right though, write-caching does need to be enabled.