EDITED !!

Don't worry about latencies with an PCIe adapter in general, assumed that it has a good driver.

I would recommend to get a working USB3 interface and use it in USB2 compat mode.
Makes no sense to sacrifice a "precious" PCIe socket on mainboard for an USB2 controller.

Not 100% sure, whether USB2 compatibility mode could have issues under different USB3 chipsets...
As BBF Pro is an USB2 interface there is no comment in the manual in regards to compatibility to USB3 chipsets.
I am running i.e. the ADI-2 Pro which is an USB2 interface also on an USB3 card in USB2 compat mode.
Something like this should also work for you with the BBF Pro. You need to try.

With the madiface driver for UFX+/II etc I can recommend cards with FL1100 USB chip.
For Windows 7 they offer good working drivers and under 10 the support comes with the OS.

The advantage of the FL cards driver is, that it uses MSI's (message signalled interrupts).
This is a more efficient interrupt handling method.
Such cards with FL1100 USB3 chips are backward compatible to USB2, so also devices like ADI-2 Pro or
Octamic XTC in Class Compliant mode work without issues.

I would start with such a card and try whether it also works nicely for the BBF Pro (which uses another RME ASIO driver than madiface driver).

I am using a Sonnet Card, currently there is only the cheaper card available, which does not have an USB3 chip per port (4) but one FL1100 USB3 chip for all ports. I would use this card then only for connecting the recording interface, to isolate it from other USB devices.

http://www.sonnettech.com/product/alleg … 4port.html

EDIT: if you look for this card, ensure that you get the model with 4x USB3 Type A connectors, there is also another model with type C connectors, I would use the 1st one, the other might be a different card, am currently checking.

Comments:
I personally could connect two UFX+ (lots of channels, 2x188=376 !) and one ADI-2 Pro to a Sonnet Allegro Pro on Win7&10.
This particular card was quite expensive as it had an USB3 FL1100 chip for each of the 4 ports dedicated.
Therefore the price was quite high at around €150.
Now this card is not available anymore, the new Allegro Pro is a different card with only two USB 3.1 ports and a different USB chip where it is not sure, whether it works well and how backward compatibility to USB3 and USB2 actually is.
I would simply try the old Allegro Type A USB 3.0 PCIe 4-Port.
You can download either the Win7 driver from Sonnet: current version 3.8.35514.0
Alternatively directly from Fresco Logic.
I had no issues with these driver versions under Win7: 3.6.9.0a, 3.8.34556.0

How do I know which versions have the original trusted chipsed that you've mentioned?

In Germany its this card:
https://www.heise.de/preisvergleich/son … 48037.html

You can potentially differentiate it by this product description: USB3-4PM-E

Amazon Germany seems to have still has two offers of the old PRO card with 4xFL110 chip, which requires a PCIe x4 socket (!!!) https://www.amazon.de/Sonnet-USB3-PRO-4 … amp;sr=8-2
But with €172 its very expensive and you need the x4 PCIe socket.

This is the less expensive card with 4-ports and only 1 FL1100 chip which only needs a normal PCIe x1 socket:
https://www.amazon.de/dp/B00GRGCV2G?lin … DlYialxarQ
At Amazon DE this card costs a little over €70, cheapest price in DE around €60.

I assume you stumbled now over the newer model with two USB 3.1 controller, this I do not mean.
You need to look for the card with 4 USB3 type A ports.

This is the new one with two controllers, rght? You've saying if I get the old one with four controllers then I can run 4 devices off the card versus Babyface only off a single card linked above?

So if I get this card :
https://www.amazon.com/Sonnet-Allegro-4 … amp;sr=8-2

- I should dedicate the whole card to a single Babyface Pro, right? And I should run all other USB devices off another identical card or cheaper no-name Chinese cards, right?