Is there a way to bulk edit the custom excerpt text without going to each post and open it? When we have 300 posts it can be a very time consuming process.
Search RegEx is a good plugin to be able to search and replace (optionally with grep) through all posts, pages, excerpts, comments, titles and meta.
Are you looking to edit each post's except in bulk or via quick edit?
If so, I wrote a plugin to do that, see http://wordpress.org/plugins/custom-bulkquick-edit/.
Be warned, this is only for posts and pages. If you want to edit excerpts for custom post types, then you'll need to purchase the premium version.