I tell You what happened with me
The tests made R9 280x  -I 18 -W 64 without any -g parameter gave me 51 mH
then I add at the end -g 5 and have 70  - 71 mH  - then set -g 3 and had 72 - 72 mH
I tried other combination of the 2 values but  (-g 3 and -I 18) is the winner
You decide to try or not

for other cards - other values will possibly give better results