Thanks for the advice. I was looking at another thread and following the interesting discussion on problems getting the full 16x when the drive was in certain external enclosures because of inadequate IDE (UDMA) support.
I ended up ordering a Plumax PM-525C2-PPS from dealsonic. The thread indicates most people seemed to have the best luck with the Prolific chipsets for external enclosures because they seemed to be the best supported/easiest to upgrade/most likely to get 16x; while the Oxford 911 chipsets seem to be the most unlikely to work and there was little to no flash upgrades to fix the problems.
For those of you that suggested other cases, are you getting 16x? What's your chipset?