I guess its confusing for some of us to hear that someone wants to maximize street performance, but starts with something not meant for street performance...(Silk purse - Sow's ear, etc...)
If I want all out off road performance, I don't buy an X, sorry, my Jeep kicked its ass...but, couldn't carry as much as I needed, so I had to compromise, and get an X, and modify it....and I'm happy with the compromise.
If I wanted to autocross, then the X would not be a compromise, it would be stupid, there is nothing reasonable that could overcome its weight and other handicaps....I would start with a large enough sports car maybe, and modify from that platform if I needed carrying capacity, and on-road handling.
So - its reasonable to try to find a starting point that covers your needs, and can be molded into a reasonable objective...it just seems weird that some people choose starting platforms that will never be able to be modified to reach their objectives.
After that, its all a question of degree.