They do not have to provide an OC service, its an option for those who are not comfortable OCing on their own. I would imagine that there isn't a 30% option because they do not want to be responsible for anything for than 20% or at least they wouldn't warranty a system past 20%.
All you need to do is do a little research on the forums here on Tom's and you should be able to OC yourself pretty easy. Newer Intel and AMD systems are worlds easier than it ever used to be.
You can buy GPU's that are "factory Overclocked" but no GPU will OC itself directly.... If you want to ramp up the frequencies on your GPU you can download a program called MSI Afterburner it's pretty self explanatory. I wouldn't recommend unlocking the voltage on your GPU though, just work with-in the stock volts.