Yeah the RE4's should be doin better than that. Probably a controller issue but I'm guessing.
Since they are RE4's, I don't think they're prone to the TLER foolery found in non-enterprise drives. In that case, have you tried RAID 6?
Thick pre-allocates the space. Thin dynamically expands.
When you get your P410, make sure you get a cache battery and memory chip. I paid around $120 ish on eBay last week. I got mine from this guy: HP P410 SAS RAID Controller 256MB 462919 001 013233 001 w Cables Battery | eBay