Modern headers flanges are usually surface ground straight, I learned to check every set.
The remflex helped seal the old style headers where the flanges were far less than true, and needed the high welds filed down, and valleys welded up, then draw filed to get them "true".
I learned after mosta the first headers I installed came back cuz of leaks or rattles.
My mentor made me "check and file" every set, dozens over the years.
I've used the gaskets that come with the headers for the last few dozen sets, never a problem cuz they are "true".
Never ever used a "Remflex".
Regular Mr Gasket otherwise.
Good luck.