The Atom and 7 feature the same 2.4L V8 which gives them both more than 1,000 bhp per tonne to compare with the Caparo, and I added the T-rex and X-bow just for good measure.
Atom: http://www.supercars.net/cars/4203.html
7: http://www.supercars.net/cars/4078.html
X-Bow: http://www.supercars.net/cars/4535.html
T-REX: http://www.supercars.net/cars/290.html
T1: http://www.supercars.net/cars/3376.html
I may be leaving some heavy-hitting lightweights out, so if you think of something please bring it in to the discussion!