unfortunately I don't think you can just buy one and drive it in California - Is it legal
if you read that link it tells you the process needed to get one to be street legal in CA - - - considering this and the price you would probably be better off throwing a quad kit on your slingshot along with a DDM supercharger - - - probably even save some money
You can actually get it road legal from the factory. I have 2 friends that have bought them that way.
As far as doing a quad kit and boost...I did that. And I drove both back to back on a track. It is a completely different vehicle. The quad gives you good stability...and the 2.4L with the Hahn turbo gave it good power...but the Ariel Atom is more of a surgical instrument. In the proper hands it can do amazing things. The Quad with boost slingshot is lightyears away from an Atom. It's the reason they get the asking price and then some for them.