AFAIK, the largest tier in any AWS instance type has always been the full box. i.e an m1.xlarge is the whole box, an m2.4xlarge is a whole box, etc.
> Why the range? Write IOPS performance to an SSD is dependent on something called the LBA (Logical Block Addressing) span. As the number of writes to diverse locations grows, more time must be spent updating the associated metadata. This is (very roughly speaking) the SSD equivalent of seek time for a rotating device, and represents per-operation overhead.