I have serious doubts about this claim. An Ariel has 500hp, weighs slightly more than half a ton. It does zero to sixty in 2.3 seconds (a full second faster on than the MP4-12C) and can out-brake a fly. I know it has understeer issues, but I don't think the MP4-12C can make up for that.
I know that these may not include the optional lightweight pack and the corsa tires, but the difference is still profound.
Specs:
McLaren:
Power: 592 horsepower
Weight: 1,300kg
P/W: 455hp/ton
0-60: 3.2sec
Atom 500
Power: 500hp
Weight 550kg
P/W: 900hp/ton
0-60: 2.3sec
That's just a completely different league...