Wifi Antenna in Mac Pro 2008 (3,1) specs

Hi,

I installed an Airport Express Wifi adapter (BCM94331CD) and Wifi and Bluetooth work OOB. running El Captain 10.11. The Wifi adapter should be able to do 450 Mbps according to the specs, however, I only get 300. I have a Mac Mini 2012 next to it which does 450 no problem. So it's not a problem with my Wifi router.

Below is how I connected the cables:

Is there perhaps a speed limit of the Wifi antenna in the Mac Pro? If I understand correctly, the 3 wires are 3 antennas, which should be able to achieve 450 Mbps. Can anyone confirm this?

Or perhaps the Wifi adapter is bad? The connection is also constantly dropping and I have to reconnect.

Thanks.