I am using a Gigabyte Z270-HD3 motherboard with a USB 3.1 2nd generation add-in card. A Samsung T5 SSD connected to a port on the add-in card perfarms at above 500 mb/s read and write (Crystaldiskmark)
Would there be any real advantage in going to a Z390 board?
Keep in mind 1) your MB can support 6th and 7th gen cpu only. 2) the Z390 will support 8th or 9th gen cpu. 3) The samsung T5 SSD spec for the reading and write speed are: Sequential Read up to 540 MB/s and Sequential Write up to 515 MB/s.
So if you spend the money on the new cpu+ z390 for less than 1% to10% faster, I don't think it is a good idea.