While I'd agree with the speeds, when referred to in a theoretical sense, an Oxford 911+ enclosure will hit 18x. I've done so with an LG H22N. Ok, it was 17.9x, but keep in mind it was connected to my laptop docking station. Hook it up to a desktop with 7200 rpm drive and I have no doubt it'll hit 18x.
Since the 1.06 firmware update to the 760A, I'll have to retest it on FW in my external enclosure. I also have one of the Neo enclosures equipped with an NEC USB chipset. I've seen where Archane has hit 18x with a Sammy external and it has the NEC.