Using the LPC4357, it is possible to run the main clock at 204 MHz and external SDRAM at 102 MHz. Emcraft has validated that these clock settings work reliably using various custom LPC4357 hardware boards in several consulting projects we have developed for our customers.

With the Embedded Artists Dev Kit hardware, we had to limit the main clock to 144 MHz and the external SDRAM frequency to 72MHz. This has to do with the following errata of the Embedded Artists LPC4357 hardware platform:

http://www.embeddedartists.com/sites/default/files/support/oem/lpc4357/LPC4357_OEM_board_CLK2_OUT_Note.pdf

This is an unfortunate limitation for uClinux since, as described in What is the minimal footprint of uClinux?, uClinux runs from external SDRAM. It has been our experience that using the LPC4357 the overall uClinux performance is pretty much proportional to SDRAM frequency. The implication here is that you are able to run your custom LPC4357 at 204 MHz / 102 MHz (main clock and SDRAM, respectively) you can expect peformance and boot-time numbers to improve in a more-or-less 1.4 proportion, as compared to what you see on the Embedded Artists LPC4357 Dev Kit.