Glad to hear it worked. I've been plagued by the internal Bluetooth too.
Up until today I had disabled the internal 2.0 and was using a Bluetooth 4.0 dongle attached to the USB port of my Apple Pro keyboard.
This morning I installed the Wifi AC/Bluetooth card that has been used in another thread to add Continuity/Handoff compatibility to the cMP.
Following the OP antenna arrangement resulted in the same poor Bluetooth signal despite me having replaced the Bluetooth antenna with one of these
http://www.osxwifi.com/wireless-internal-antenna-for-bluetooth-macpro and attaching it to the front grill.
There was a post here
https://forums.macrumors.com/posts/19389239/ outlining the use of a built in Wifi antenna for Bluetooth along with the addition of another antenna. I left the new antenna on the front grill using the same principle and have changed the order of the antenna connectors so now from top to bottom it reads 3,1,2,BT. (So now the Mac Pro detect Wifi using the antennae 3, 1 and the antenna on the front grill. Antenna 2 which is in the Mac Pro Wifi cluster is now being used for Bluetooth instead. I hope that makes sense)
Monitoring the connection quality using Hardware IO tools shows that it can range from 58% to 100% but being honest it largely sits at 80-95%. It only seems to drop to 58% transiently if I grip the mouse really hard. I cannot detect any lag and now have wake from sleep by using the Apple Mouse

I'm a very happy camper.
I've attached some photos of the setup. You can see the antenna through the front grill but I don't care as it's an internal solution that seems to be working. The antenna was easy to thread into place and the only component I had to remove was the CPU tray so all in all the most difficult thing was connecting the antennae to the card.