Using USB 3.0 to speed up transfer between USB 2.0 devices?

According to the USB3 specification from here, USB2 functionality on USB3 hosts/hubs does not change. Therefore, (putting power issues aside) USB2 devices still operate with a broadcast method, meaning it will share the same old USB speed bandwidth with all other USB2 devices on the same host/hub. USB2 devices will not have USB3 capacity available to it, as the SuperSpeed USB3 capacity is on different wires that are not connected to USB2 devices.

Also, keep in mind each USB port may or may not be it's own host, depending on the hardware manufacturer. Sometimes they will have one host for each port, and sometimes one host will manage multiple ports. To find out for sure which hosts manage which devices, open up Device Manager, and click View -> Devices By Connection. Open up the "ACPI" devices, and then there should be a PCI Bus device under that. All of the USB Host Controllers should be under there. Try plugging the device(s) into different ports and see which Host Controller it appears in. Sometimes a Host Controller won't appear until there is something plugged into it.

USB3 SuperSpeed device's data transfers should work parallel to a USB2 device as it uses a different set of wires, and likely would not conflict or slow down any USB2 devices also working off of the same Hub/Host aside from maybe a little handshake talk when the device is first plugged in.

The xhci specification clearly states that an individual controller may support multiple "bus instances", each representing a bandwidth unit, e.g. 480 mbit for high-speed. See the second and third paragraphs in section 4.6.15. The example provided there is 1 SS + 2 HS + 4 LS/FS for 7 distinct BIs of bandwidth divvied up between 8 physical ports. I'd love to know whether any shipping hardware implementations go the extra mile to implement it. I haven't been able to find explicit mention in the documentation for various chipsets. Given that superspeed-to-highspeed transaction translators are conspicuously absent from the USB3 spec, it would seem the best way to support a large complement of bandwidth-hungry USB2 devices.