Possibilities to increase overall USB 2.0 devices throughput with PCIe expansion card

Question

I've checked most of the U3/U2 bandwidth questions on-line, but still have some confusions.

I understood that, if I had one USB 2.0 controller and one USB 3.0 controller in my laptop, even if I plugged my two bandwidth-consuming USB 2.0 devices (both needs 250Mbps) separately to both controllers, the two U2 devices still share the Hi-Speed (480Mbps) from the USB 2.0 controller (xHCI).

If I added one "PCI Express USB 2.0 expansion card" with 2 ports (dedicated controllers), do I get total "3" or "2" or still "only one" Hi-Speed (480Mbps) throughput?

If I added one "PCI Express USB 3.0 expansion card" with 2 ports (dedicated controllers, e.g., μPD720202), is it helpful for my scenario? Can I get both U2 devices with 250Mbps, one plugged to the original laptop U2 port and ont to the U3 expansion card? I'm asking this is because I saw these kind of U3 expansion card claimed with the compatibility of USB 2.0/1.1/1.0, and I'm wondering if it can act as an independent USB 2.0 controller, or just route to the xHCI of my laptop.

Additional question: If I disabled the built-in xHCI of my laptop, and just use such PCIe USB 3.0 expansion card, can I still use my USB 2.0 device with its port? If the answer is yes, and there are two ports (with dedicate controllers, let's say μPD720202 *2) on the interface card, does it mean I can plug both my 250Mbps devices to it as it being two independent xHCI ports?

Thanks!

Answer 1

Old classic EHCI and many newer xHCI controllers have a bandwidth limit of one 480Mbps channel per CONTROLLER. So having two controllers will provide two HS devices with 480 Mpbs for each one. More controllers should do more, provided that the internal PCI interconnect fabric is not saturated.

Modern Intel xHCI controllers (2015 and newer) have an ability to stream full HS bandwidth (~42 MBytes/s) on each downstream root port, see this updated result.