They're right - you can't use 700 properly on the street, but that doesn't mean you shouldn't do it. I had a 400hp Fairlane street car about 8 years ago. With skinny BFG T/As it was like driving on ice and it scared me more than a few times. It definitely needed another 300hp.
Regarding inkjunkie's comments on bends and strength, it's common to use that kind of design over here, even in our top-level roadrace cars ( NASCAR equivalent). There is probably a whole bunch of engineer mumbo jumbo about how it is done though.
The best advice is to find a good cage builder, take the car to them, talk through your plans and then put your life in their hands.
