[Tommy]

Dirk,
As I read your initial post, your question boils down to this - What are the advantages and disadvantages of building and operating an ultralight car? Here's my answer in list form.
Advantages
1. Experience the satisfaction of taking on and completing a very challenging task (i.e., conceiving, engineering and building such a car).
2. Experience the pleasure of sharing your building experience with other car people after the job is complete.
3. Enjoy the exhilarating and humbling experience of trying to master such a car on the track. (I don't see street driving being much different than in any other powerful Cobra replica. Most of them can scare me silly, ultralight or regular.)

Disadvantages
1. High costs for one-of-a-kind engineering, material and parts.
2. Potential safety compromise if you don't properly engineer and construct the car to achieve the ultralight weight.

If it were me, I would see lowering the weight of the car as a means rather than an end. I wouldn't invest the time, money and effort to build an ultralight car unless I had in mind a specific end use that would justify all that effort.