Interesting.
Found this thread from the past where Chris_seven (it seems he also makes head studs for the 911 and whose posts appear highly technical) says
"ARP, Supertec, Casper Labs and any other stud with an increased shank diameter will increase the force on the thread produced by the thermal expansion of the cylinder."
http://forums.pelicanparts.com/911-engine-rebuilding-forum/790482-arp-head-stud-installation-instructions-vs-reality.html#post8453631
and has the same question as I did above about the need for the forming tap.
"The forming tap is an interesting idea do you know the class of fit after using a forming tap? I would guess that this will open the thread slightly and increase clearance which may make it easier to install the stud."
http://forums.pelicanparts.com/911-engine-rebuilding-forum/790482-arp-head-stud-installation-instructions-vs-reality.html#post7842555
I suspect this is water under the bridge stuff in 2018 but I will keep looking for that side by side physical comparison of the Supertec vs OEM thread.