Your performance sounds about right. The P812 is a 6Gb card, and you're getting consistent 4Gb performance in a RAID10 configuration. Pretty strong, especially with only one enclosure and one pair of SAS channels in use. It shows both channels are actually being used, otherwise your performance would be closer to 375MB/s.
In order to get more performance, you're going to need more than one D2700 enclosure, and run them off of the second pair of ports on the P812 card. Set each enclosure up as a RAID0 LUN and then mirror them. That way, your mirroring I/O won't contend with each other and your throughput should increase significantly. You may not get much past 1GB/s though
The bigger question is what kind of I/O are you expecting this system to handle? You say "lots of I/O performance", but there are a couple of ways to define that. You seem to be focusing on simple throughput, but your choice of disks suggests latency is actually a major concern.
I'd suggest characterizing your storage performance across a variety of access sizes to get a better feel for its overall performance. If you know what kinds of disk transfers your high I/O application needs, focus especially on those ranges. Also pay attention to stripe size on the RAID sets and where your partition-breaks fall. This is more typically an SSD concern, but you seem to want max-possible performance so the extra percentage points you get for ensuring your filesystem blocks align with the RAID stripes is worth looking in to.
Simple file-copy is not enough to characterize the performance of a storage system. For that you need real benchmarks. I'm particularly fond of IOZone (link), but IOMeter (link) has better market penetration. Focus your testing on data-sizes you're likely to be working with and I/O transfer sizes you're likely to use. It can be very amazing how different storage performs when working with 4KB chunks and 32KB chunks.