A self enhancing human will still be hugely slower than a self enhancing AI. Compare the progress in computing power and software to the progress in human prosthetics. Computers don't die if you plug in a new RAM card (unless you're very unlucky). You can run a newly optimized algorithm on a computer in a way that you just can't do on an organic brain. If we get to a point where "humans" are as easy to optimize as computers then they won't be humans as we know them anyway.