i just came across the X1 prototype created by adrian and its almost 20 seconds faster than these f1 cars so as a single seater that would be the pinnacle, it included a fan like the one used by brabham bt46b, and a gas turbine engine. heres a brief overview
"designed with pure speed in mind, rather than to regulations. It is therefore far superior to a Formula One car in terms of speed and handling. Its extreme specifications are hypothetically capable of taking 20 seconds from the current F1 lap record at Suzuka. It was said to have a top speed of 249mph and weigh 545kg with around 1509bhp. After much tuning the X2010 was capable of achieving a top speed of 307 miles per hour. The downforce must be turned down to achieve the max speed."
and heres a link to see the beast:http://www.youtube.com/watch?v=dmtehAZV ... ature=fvwp
Now thats a diffuser