Here's how the estimate for 60% greater performance density (IOPS per rack unit) was calculated.
A NetApp storage system configured with the DS4243 disk shelf with 16 x 15k RPM, 3.5" SAS disk drives as tested under an OLTP workload. The number of drives was chosen to ensure that the storage subsystem, not the controller, would be the performance bottleneck. Data was collected plotting latency (ms) as a function of the load (IOPS). Then the same test was run with everything the same except the backend was changed to a DS2246 shelf with 16 x 10k RPM, 2.5" (SFF) SAS disk drives.
A plot of the results showed that the 10k RPM drives provided 78% to 82% of the IOPs of the 15k RPM drives across a wide range of fixed latencies. Hence, the 10k RPM drives provided ~80% of the performance of 15k RPM drives.
Performance density was determined by factoring in the rack space needed for each set of drives. The DS2246 disk shelf packs twice as many 10k RPM drives into the same space as the DS4243 shelf with 15k RPM drives. So, 80% of the performance (IOPS) in 50% as much space (rack units) translates into 60% higher performance density.