HP's published drive specs on their SAS drives seems to indicate there is a difference in the seek times on the drives:
HP 146GB 3G SAS 15K 3.5" DP ENT HDD
Transfer Rate Synchronous (Maximum) 3 Gb/sec
Seek Time (typical reads, including settling)
Single Track 0.57 ms
Average 3.5 ms
Full-Stroke 9.0 ms
HP 146GB 3G SAS15K SFF (2.5") DP ENT HDD
Transfer Rate Synchronous (Maximum) 3 Gb/sec
Seek Time (typical reads, including settling)
Single Track 0.14 ms
Average 2.58 ms
Full-Stroke 4.85 ms
So 2.5" seems quite a bit quicker on the seek times. You also wanna be mindful that the new drives come with the 6G SAS interfaces which are more rapid then the 3G interfaces, but the disk controller has to support it (such as the P410's you get in HP DL G6 servers after a firmware upgrade).
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.
Best Answer
You should be concerned with head seek time and transfer rate. It is true that they depend on the form factor, but they also depend on many other variables. Looking only at the physical size and ignoring those variables would be wrong.
With this in mind, let's compare the most recent versions of some widely used disks: Seagate Cheetah 15K.7 and Savvio 10K.3.
Bottom line: 2.5" disks are comparable to 3.5" disks for random operations, and usually you can compensate or even win with 2.5" due to the larger number of spindles in the same physical volume (disk array or server). However, if you need to pump a lot of sequential data, 3.5" disk is still the king.