The highest performance 8" sub in existance of which I am aware is the JL Audio 8W7. It DOES produce the output and distortion profile of a very good 12" normal type subwoofer. It has a linear excursion of 2" peak to peak, with no discernible distortion or compression occurring. That's 20% of it's diameter! It can actually manage about 2.7" or so peak to peak, but it's audibly distorting at this point. It has massive thermal capacity. It's motor and suspension is extremely linear.
This driver can be used in tiny ported systems that have insane output levels. If you need a very high output/high performance sub in very compact sizes, this can fit the bill. A gross volume of about 1.8 - 2 feet should be enough to get a ported box with incredible output, exceeding what very high quality typical 12" systems can produce, and that are far far larger. You could pull it off for about $650 total cost, DIY. Ask me about it.
You can get it for about $250 plus shipping from discount web dealers. Or pay over $400 from an 'authorized' JL dealer.
TC Sounds used to make a comparable driver, but TC is out of business now.
-Chris