I have grill grates on two Webers and I love them. I broke one in with Pam and not nearly as good a break in as the other one that I broke in with coconut oil.
I now use coconut oil on both and I couldn't be happier. More heat, great grill marks, great grilling, no flare-ups, easy cleaning.
In both cases I completely removed the existing grills, rather than place them on top!
So far I've grilled beef tenderloin filets, New York & ribeye steaks, Pork steaks and chops, Pork tenderloin, chicken breasts, salmon, sausages and vegetables, ie peppers etc. Coupled with a Thermapen thermometer, I've had no failures!