you can only get so loud with one tweet like i have and a 5.25 in the factory location
some people would be content with it, but me, not so much
If you want to go very loud I think you should go for the Digital Designs VO-M6,5 or perhaps the 8 inch VO-M8. With the sensitivity of 98dB for the 6" you could go for 122dB for one speaker at 256W, +3dB when doubled as a pair. Loud enough to get permanent hearing damage and they go for an easy fit in the stock location to. Eights can be fitted behind the door panels in both OG9-5, NG9-3 and newer SS modells.VO-M6 : http://www.ddaudio.com/products/mobile-audio/specialty-drivers/vo-series/vo-m65.aspx
VO-M8 : http://www.ddaudio.com/products/mobile-audio/specialty-drivers/vo-series/vo-m8.aspx
If such results are not good enough I think one need to build new door panels with multiplied speakers. I would put the speakers in front of the door and low. For my own car I recently bought four tens, which theoretically should be able to produce 132dB at 512W before cabin gain. As a try-out.