I did suspect the USB 3.0 would be useless, but I did this instead: Lexar 4GB key as the ESXi Host Image (I have used these Lexars for years now...work great!) [have to use the ks.cfg like you did] and since I could use the onboard storage controller now with that VIB, I can use a RAID 1 mirror with Seagate hybrid drives (1TB) and host many VMs from there. This is intended to be a router/firewall and file server for use in a home. I am thinking about trying out different USB 2.0 PCI cards to see if they work too.
How does it perform for you?