[presumably the empty-disk effect could also be achieved by partitioning, say 25% of the drive for the database, and 75% empty partition. But in fact, you could use that "low performance 75%" for rarely-used or static data, such as the output from pg_dump, that is written during non-peak times]
Larry Ellison financed a company called Pillar Data Systems which was founded on the principle that you can tier the disk according to the value and performance requirements of the data. They planned to put the most valuable in performance terms on the outside of SATA disks and use the empty space in the middle for slower stuff..
(This is not an advert. I like the idea but I dont know if it works well and I dont have anything to do with Pillar other than EnterpriseDB compete against Larry's other little company).
Probably the way to go is flash drives for primary performance data . EMC and others have announced Enterprise Flash Drives (they claim 30 times performance of 15K disks although at 30 times the cost of standard disk today ). Flash should also have pretty much consistent high performance across the whole capacity.
Within a couple of years EFD should be affordable for mainstream use.