No question that you'll see a real improvement over a 5770. Couldn't scrape up the cash for an actual Mac Edition, so I'm running a flashed MSI Twin Frozr 7950 in my 4,1 that replaced the 4870. 10.9 has been a solid upgrade for performance. The little flickering I saw in 10.8.4/.5 are gone in Mavericks. The only sad thing remains the lack of support for Bioshock Infinite. Soooo pretty and fast in Windows. Soooo sloppy and unplayable in OSX. Some day... My Tempo eSATA fizzled with 10.9, but reinstalling the drivers fixed it. For now, at least.