Will submitting to the Yahoo directory benefit in any way, my SERP on Google? Is shelling out $299 worth it anymore, especially after the last update?
Yes submitting to Yahoo directory will have a great effect in all SERP's. Any backlinks from highly authoritative site will has a great value in SEO.
Yes, but not significantly. A link from Yahoo directory is just one link. It will not give any big effect on your SERP.