Hello,
Welcome at the VMware community forums.
The issue you are referring towards is about a guest OS directly connecting to a USB3 disk, aka you plug the usb disk into your Mac and the guest OS (Windows 7) gets direct access to the USB3 disk.
What you are talking about is using a Virtual Machine that is stored on a USB3 disk.
The guest does not know where it is stored as that part is completely abstracted from the guest OS.
There's no problem with using USB disks for locating your VM on there.
If your Mac recognizes the USB3 disk then you can store your guest on there.
Using an external disk like a USB or FireWire disk is a reasonably popular way of using virtual machines as it does not interfere with disk IO as needed by the host.
I cannot give you exact figures on how well it compares to your SSD disk as there are many factors.. including your SSD disk performance.
But in general using an external disk does work well.
Hope this helps,
--
Wil