Any performance impact or reason to delete unused views? I have maybe 250 views on a site build and estimate that I use around 150 of them.

Is it ok to leave the extras there unused? It would take days trying to guarantee views aren't being used in custom blocks and in hooks for other views.
If they are never loaded/rendered, there should be no performance hit, except for the admin viewing the /admin/structure/views page since they get listed there.

Ok, maybe a slight performance hit, since the view table ID's in your DB is bigger, so it would take longer to find an ID, but if you're caching your Views you will be fine.