I have just successfully installed a Chinese 802.11ac/BT4.1 kit in my Mac Pro 5,1 but I encountered a weak BT signal issue in the process and for those of you who are having the same problem (jaggy carsol movement on the screen and frequent signal disruption and all) but don't want to read all past topics on these forums to find out the basic remedy I want to share what I did to fix it. It's easy.
Connect BT cable to port 1 instead of port 3 and connect WiFi 1 cable to port 3. The rest of WiFi cables can go to port 2 and 4. That's it.
As I remember in my old Mac Pro 3,1, I had a similar problem when I installed 802.11n/BT2.1 card and I swapped cable 1 for cable 2. These thin wires are antenna cables and they are attached to different parts of Mac Pro's aluminum body and depending on where they are attached on the other end the signal strength varies. Very simple to understand but very difficult to find out for us novices because this antenna swapping technic is assumed to be a common knowledge and does not get mentioned too often in these forums.