js, you might be underestimating the amount of work involved to put a taller windscreen.
I am not sure if the roof shell needs to be changed but if it does, it is major work, BTDT. The A pillars in the front are definitely changed, I do not think the roof was lowered (ala Speedster) it might be the visual effect of the longer windscreen, therefore the side doors and rear quarters are standard size.
Despite the picture below, the height is reported as standard, perspective makes it look different in this picture..
The windshield was built by DP Motorsport and was also used in another 930 creation, also the front hood comes from DP if I am not mistaken. I am also not sure that the 911&Porsche magazine article is accurate, I believe this was a 993 body to begin with not a 964, and it is a narrowbody with shaved rain gutters as says the article.
It has the 964 4WD system in it instead of the 993TT 4WD system, and a 964 5 speed gearbox that has been customized with 1st: 3.5, 2nd: 2.11, 3rd: 1.44, 4th 1.02 and 5th: 0.75 (same as top end of 993TT).
Notice the front tires' wheelspin?
The reduction is CDxA must be substantial, probably close to RUF's Yellowbird.
All of the above combined, a 7500RPM redline, front windshield, narrowbody, gearbox, strong engine are what makes this car so unique. Imagine that with another 150HP.