My Nspire Oxford 911+/Cypress AT2+ external case will give me 18X burns on the Firewire side and true 16X burns on the USB2 side from my 3 year old Toshiba Laptop. The Nspire case houses an LG H22N_1.02.
My USB burst rate is 19 Mb/sec on a burned disc.
And Yes, you are probably right about USB Host Controllers affecting your results.
All in all I would say the external case you have is very good. I think I will have to seriously consider getting one of these.
Thanks for your input.