OK lets get technical. Bandwidth is a term used to describe the amount of data that can move in a given period of time through a conductor. In the case of a GPU the data generated by the CPU that represents what needs to be graphically displayed is sent along a bunch of conductors "PCI Lanes" to the GPU which in turn converts that data to a stream of data sent to the display. So in this case 5GB of data per second is sent to the graphics card. With PCIe x16 you could see a maximum theoretical bandwidth of 8GB/s, so your adapter is decent. Any update would be any increase in performance and possibly a decrease in power consumption (unless you go crazy).
Thanks for your great fast response! I guess it's kinda impossible to calculate the bandwidth stuff. Even Nvidia is unsure about any bandwidths. Just to add, I will write my theory here: Computers reansfer electrons (Voltage and current) so I believe I need to choose as much power efficient card as possible while keeping quite a decent one. Fallowing the formula P(watts)=I(current) * V(voltage) will confirm this. In the end since my whole GPU potencial is only 62.5% ( (100% / 8)* 5), various videos comparising desktop and external GPU confirms that the External GPU FPS results actually are about 35% worse. Thanks anyway and I hope it will be helpfull for everyone