My burst on the enclosure above was around 21 mbs USB2, and 26mbs for Firewire. But burst doesn't really tell you more than it may be able to reach that speed for a second. It the sustained transfer rate that is critical.
I have a older ADS oxford chipset enclosure and it shows burst of 24mbs using usb2 but trying to write above 8x it fails, it is pretty much useless other than 8x.
The reason for the ADS failure is a issue with external enclosures and how they handle the ATAPI interface(the same issues are prevelent with certain PCI IDE controllers), my ADS even with the newer oxford chipset uses multiword2 dma mode on many of my writers instead of UDMA, pretty much limiting the speed. So sometimes it is a combination of a good chipset and a good atapi interface that handles udma modes well with ATAPI devices.