My Challenge to U /6ers....

In your original post, one of the requirements was, "it can keep up w/ a Camry, for example."

I just watched a stock, showroom, 2011 Camry beat a 308 Ferrarri (like the one Magnum P.I. used to drive on that TV show) in a quarter-mile drag race on a syndicated program called "The Car Show."

Ever since the Camry received the Avalon 3.5-liter V6, in about 1996, the magazines that road test them usually report quarter mile times of 14.40s at nearly 100 MPH (99+). A Nissan Altima V6 is even faster...

A normally aspirated slant six that will keep up with a new Camry will cost you a lot more than what you're suggesting.

You COULD do it with a turbocharged slant six, but you'd have to know what you were doing... and get some good deals on parts...