The bobbins with rubber infusion don't flex any more than the plain ones, but I can feel a difference when sliding a finger over the surface of the bobbin. The rubber-infused ones are not quite a slick, which is the intent. It makes the bobbin a little more "grippy" in the bobbin case.

The prohibition on using metal bobbins in a top-loading Janome machine stems from the magnet holding the bobbin case in place. Plastic bobbins are immune to the pull of the magnet, but metal bobbins could drag a bit, resulting in slightly increased bobbin tension. Over the years I had MANY customers that used metal and plastic bobbins interchangeably, most because a particular bobbin might already be wound with the needed thread color. None had any machine problems as a result. A metal bobbin won't make your machine blow up, and it may not produce any effect you can observe. However if you have a vast collection of metal bobbins salvaged from great-grandma's box of sewing stuff retrieved from the barn, and they are coated with a generous patina of rust, chuck them out!

And while we're on the subject of bobbins, let's address those precious "rainbow" bobbins in your sewing cabinet. You know, the ones that have 5 different colors of thread wound on them because you couldn't find an empty one when you needed it. The amount of money saved by doing this is almost certainly not worth the time lost fiddling with them. Money spent on empty bobbins is always a good investment!