You're right about it being pointless to tune further once acceptable performance is reached. 1.5 dynamically generated hits/second (I am assuming most of the 17 hits/second are static content such as images or CSS) is abysmal performance on a machine such as yours, however, by at least an order of magnitude, even when using slow languages such as Tcl or Python. There might be a configuration error such as using too few database connections.

You have to keep in mind any geek website could be slashdotted. In that event, even a 20x safety margin is likely not to be enough. That's why I generate static HTML pages, Radio-style, for my own blog from a Python-based tool rather than serve them dynamically. Sure, I lose the ability to host comments, but as those are abused by spammers anyway, not a big loss compared to the added robustness.