Greg, I think you are right only to a certain degree. The whole factor is $cost$. With supercharged, turbo set ups it is completely possible to get deep into the 9's and be classified as (perform)"a true protouring car". Look at Tom Nelson... This guy is insane (in a good way) and makes his living on doing engines & PT cars (as well as others) that can have insane street manners, incredible handling & braking and big power on the track. A t56, heat treated & cryo treatment if built correctly can handle well over 1200hp& 1000ft lbs, but again "cost". A 4L80E with a gear vendor overdrive and a paddle shift can handle 1500hp if done right.
Now comes the factor? what's the budget limitation & time? There is so much to offer in the aftermarket out there it can be done and well done easy... Fesler, Prodigy, Gearhead and others would love someone (and they probably already have) to say that this is what they want and go build it and oh, by the way here's 300k along with it with your bonus included...
Interesting thread
Blessings,
AMS