make sure to put -i as well 16-21 i get around 18 mh/s on rx 480.
How? i have 2 gpu (RX) nd speed are 2.2mhs (both) '-'
sgminer.exe --url stratum+tcp://eu1.altminer.net:3739 --algorithm xevan-mod -u wallet.id -p c=VSX --gpu-platform 1 -I 21
pause
Try with lower I values. Higher is not always better.