Currently, your Filter Stack Inserter Plus is identical to vanilla filter stack inserters in everything except the number of filters. You could either see it as an improvement of the vanilla version -- or as a replacement. Seriously, who would use the vanilla version if your enhanced version comes at no additional cost?
I'd therefore suggest that you either replace the vanilla filter stack inserter with yours -- or that you give players the feeling the new inserter with the additional filter slots is a a technological development by making it more expensive. Just let the recipe require a vanilla filter stack inserter instead of the simple stack inserter, and lock the new version behind some research! People would have a reason to use the vanilla version (because the other one isn't available yet), and would also be able to recycle their existing vanilla inserters once your version is available.
Of course, if you feel like it, you could also implement both ideas (i.e., replacing the vanilla inserter and using the vanilla inserter as ingredient in your recipe) and let the player decide in an option how they want to play. :-)
