Just be aware that all 8x media is not created equal. Part depends on your firmware, part depends on your burner and if the media you're using is on its 'recommended list' of media that the company recommends and has tested, so they know it will work well with it.
For example, ritek g05s (Ritek Ridata 8x DVD-R) will 'overburn' (burn faster than the rated 8x speed) at 12x on a Pioneer DVR-108, 109 and the burn quality is just as good as 12x as it would be on 8x. However, there's no way it would ever hold a burn at 16x.
One MID (Manufacturer ID) like the OEM Taiyo Yuden 8x media will overburn at 16x on certain drives and actually do just as well at that speed than at 8x or 12x. However, that's the only 8x media I know of that you can do it with. I've also seen the yuden000-t02-00 mid (Fuji DVD+R 8x 50pk from Best Buy that says "made in Japan") that can also burn at 16x, but on Plextor 716a burners, the results vary; some people get very good results and others get so-so or not the results they wanted. The consensus is the best all-around speed on plextor drives for that media is 12x.
The only way to know for sure if you can get away with a faster burn is to check those things out, then make sure your burner (by using testing software like Nero CDSpeed or DVDInfoPro) can test the burn quality, so you know which media you can 'overburn' and how much. Generally, 8x media will only be a good idea to overburn at 12x, but other 8x media is only good at its rated speed. That's why it's a good idea to check your burner's website to see what media they recommend and see if they say if any of that media they recommend can be burned faster than its rated speed.