Tried similar settings, and from the other How to Mine Vertcoin thread... with my 7970/280x i still get HW errors. On an i7 3770k desktop with 8gigs of ram, I cant get above i=12... very strange.
For 7970/7990, this is the best I managed to come up with to avoid hw errs: --intensity 13 --lookup-gap 2 -g 2 -w 184 --shaders 2048 --thread-concurrency 6112