Text To Speech - Custom Rate (not on the slider)
I discovered that you can change the VoiceDataRateArray in ~/Library/Preferences/com.apple.speech.voice.prefs.plist

I just changed the value of Item 2 to a value of value 500 and rebooted.

Thanks to tyler over at https://groups.google.com/forum/#!topic/macvisionaries/HOR7NWxsDQA