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
i will try, if the -g 3 give me a totall crash i wont listen to you anymore haha

, i go check it now to add -g 3 at the end of the .bat line
Edit: Tested:
with -g 3 at the end of the line i get now: ~85 Mh/s increase around 10Mh/s
with -g 5 at the end of the line i get now: ~80 Mh/s increase around 5Mh/s compared to without -g
with -g 2 at the end of the line i get now: ~87 Mh/s increase around 10Mh/s
Still not 100Mh/s but i dont know what other guys get with an R9 290 oc tri-x (with over/underclock)
My card runs with fan speed on 50% at 68celcius, so that's not bad, it can get hotter then this but this is fine. I try to get more speed without over/underclock