When I got my Weber Performer a month or so ago, I got the Weber System grate, with the cast iron insert that goes in the center. I love the way the cast iron sears a steak. But I wish I had known about the cast iron grates you mention at the time. The Weber System insert is small, and can only do 3 ribeyes at a time. We're a family of 4, so guess who gets to eat last! LOL. MY first night with the new grill, we invited 3 friends over for ribeyes. With 7 steaks to sear, it was a kind of a pain.
I know some folks here have those grates, perhaps they'll chime in. If it were up to me, I'd say go for it!