I have not seen any info on the SoCs. Note that the Kobo and B&N Touch devices are relatively new and both use a 800 MHz clock, but the Kobo (and the iRiver HD) have a Freescale i.MX508 vs a TI OMAP3621 on the B&N. If I had to guess I would vote for the Freescale on the Kindle and (perhaps) the TI on the Sony.
EDIT: the Kindle 4 is using an i.MX50 family cpu, see
Analyzing Kindle 4.0. Probably the same on the Kindle Touch.