I am not expert on storage, however I have noticed faster write performance on the outside cylinders compared to the inside. As disk sizes have skyrocketed it seems that performance has picked up while rpm has not changed. So I deduce from this that density is the explanation.
So, why not build a disk with massive capacity to fit in the 5-1/2" optical bays most PC cases have? Performance should be amazing, even at lower rpm's, which should be good for longevity and power use. Now that motherboards and HBA's are getting past the 2.1 TB limit, and advanced format drives are here, build us a large, reliable disk to go in the 5-1/2" slot.