10k SFF drives have most of the performance of 15k LFF drives - less than 10% delta in our extensive lab tests when running tests on many LUNs from many RAID Groups (120 disks or more under load). They are the same for heavy write loads. The 15k drives are too costly to make for the volume, and will not be manufactured much longer (18-24 mths at most). Development on 15k drives has stopped, and they cannot be increased in capacity due to physics.