I didn't realize the performance dip was so dramatic. As of late I've been considering acquiring some hardware and I guess I'm going to have to keep this in mind. If ones were to buy one today I don't even think I can find stores that sell the GTX 580. Would probably need to search on ebay.
Check out this discussion - it may help you decide what card to get. There was also an email somewhere about how TITAN is currently not any faster than a 580, though no hard numbers.
Thanks for the heads up friend!