It is more like 3-4x, not just 2x. I didn't run benchmark tests; though, I wish I had now.
EDIT: Here is a link to a thread about a plug with esata enabled. See the last post (for now) for performance numbers.http://plugcomputer.org/plugforum/index.php?topic=927.0
Looking at the page I used for comparison (http://www.computingplugs.com/index.php/SheevaPlug_Performance#Update_USB_throughput_on_v2.6.30-rc3_kernel_with_mmc_patch
), it appears a later kernel fixed some of the USB slowness.
Write performance, now, appears to be ~2x and read performance appears to be ~2.5x. This is good news.
Post to the first thread to get him to try 2.5" vs 3.5", if you want those numbers. If I may ask, why do you think the form factor will influence the performance? I do realize that the form factor can infuence the raw drive numbers (smaller platter means less distance to seek); but, wouldn't the drive's raw numbers (seek, read, write) actually determine the performance?