Loud is one thing but how would it sound? Can you even hit lows with 6.5s? What kind of frequency response could you actually achieve? Seems crazy to me
It depends on the driver, enclosure, and environment. I had a single 6.5" midrange driver (not even a sub) play fine down to 40hz (and up to about 400hz) and it was loud enough off of 50 watts to be too much for a nice blending with the front stage; the output was very comparable to a single 10" JL w6AE clone I had sealed in the same car. With a better driver and different enclosure, I know I could be able to get 30-100hz response from a 6.5" or smaller driver.
I would never do it, but this is one of those things that you say to yourself, "Wouldn't it be cool to...." but never follow through with. This guy did and it's awesome to see.