Write cache on the RAID controller should help quite a bit. SQL and Exchange tend to issue lots of very small write commands. By using write cache on the RAID controller, it can batch together several smaller write commands into a better performing larger write command, along with the RAID 5 checksum.

If you look up pretty much any drive on storagereview.com, they all have pretty low 4K write performance compared to larger 2MB writes. Hard drives of any type suffer much more then SSDs, and SATA drives tend to be at the bottom due to 7,200 RPM still being common. Most SAS drives spin at 10k or 15k, helping to reduce the seek time of the drive, a major contributor for 4K write delays. Smaller frequent writes tend to involve more hard drive seeks compared to larger sustained writes.