You wouldn't hit those numbers without some front wheel help. And you would miss out on some of the directional stability and control without the ability to apply forces from that end of the car too.
Do you really need those numbers though? Why not cut down the power a little and make a more fun car to drive? The P1 is RWD and people love driving it.
At the end of the day, a roadster of that price is at minimum a second car and more than likely just an expensive toy. Make it fun.
I'm not arguing for or against RWD. But since this project appears to be a bit of a Tesla show-off (which is fine!), I don't think they're concerned with anything less than the ultimate numbers.
If it were about fun, they would have produced something about like a Miata :).
At the end of the day, a roadster of that price is at minimum a second car and more than likely just an expensive toy. Make it fun.