this could break projects where instances outside the layout is needed (simple exemple: an infinite runner where the player actually moves, which also helps a great deal with slopes compared to moving everything instead of the player), so it should be a choice by the user.
a "disable picked instances" would be better IMO than just the ones outside layout, also off screen instances aren't drawn, and if you disable their behaviors, and do what colludium suggested, they (off layout instances) won't be checked for everything put after that, and if really you need to disable them, destroying them I think is the way to go, I'm pretty sure this feature you are suggesting would actually do this destroying/recreating silently.