I have both, the grill grates work well and are very flexible in where you can place them. I find myself using the MAK grate more. It fits nicely above the flame zone and won't slide around on the grill. As for performance I think the grill grates do a little better job but I have been extremely happy with the results on the MAK grate. It is easy to clean and was designed to work with the MAK.
Here is a post where I summed up my experience with the Grate:
Whoa 3000 post BP. I use the Grill Grates more then the searing grate. The way the two are made are different. Just as BP pointed out the grill grates take the juice and sizzle them back up. The searing grate over the flame zone has a similar yet different effect. If I lost both I would buy both again.I am using the searing grate on fish and burgers now more then the grill grates.
We all have differences BP
The intent of the MAK grate is to allow air/heat to flow through the grate freely crisping the fat and still create those deep sear marks.
Because its made by MAK it fits perfectly in place of the stainless grate without sliding around on top.
Grill Grates are a good product, but are different in that they block the heat from the bottom to absorb it and radiate it through the fins onto the food. They make a direct cooking system into an indirect cooking system much like a pellet grill vs. gas. Just make sure you buy the correct size so they do not block off the airflow convection of the MAK (the BPS ones are cut to size for the MAK so this doesn't happen).
I've recently saw the MAK searing grate and I'm sure it does a great job but there are differences. The radiating effect and sizzle back effect of GrillGrates is the real secret to juicier, more tender meats. The flavorizing aspect is the 6th man in the spice department!