Vigor2927ac restricted speed
curently using WAN1 to connect to the ISP router
when i connect direct to the ISP router i get speed up and down close to 1Gb
as soon as that router is connected to the WAN1 port on the customers Draytek Download is still ok at about 800mb but upload speed seems to be restricted to about 250Mb 
before replacing the router with an alternative make, any ideas why the Draytek upload speed is so restricted, spec states passthrough on WAN up tp 1.8Gb

1.8Gbps is with two WAN connections active - i.e. marketing!

Have you got hardware acceleration fully enabled - without that it'll not get near 1Gbps.