I agree with about 85% of your post!
Just there is one thing Horst achieved pretty fast time(7.33min) with 997 GT2. Do not forget that Rohrl time is 7.32min for 997 GT2. So, in some cars Horst in pretty fast... You are right-Horst only do about three laps on the Ring. First one is usually the fastest one... Why only three laps? Well... First he picked a time when Ring is pretty empty. Second, belive it or not(I drove only once on the Ring in also loaner car) since you are Ring expert many sportscars experience brakes problems after just two laps on the Ring!
BTW, all cars that Sport Auto testes are not loaners cars.They are offical press cars directly from manufactures(and these cars are usually better then costumer cars)... Horst do not loan the car from personal owner but, Audi(for example) gives the magazine their R8 for some period of time(usually three weeks)...
Regarding R8 time... Best achivement from Stippler is 7.55min for R8 without Navi, B&O, CD Changer, heated seats etc. BUT, with Ceramics, Bucket seats and Corsas... Stippler drove R8 that nobody wants to buy if you understand me... Brilliant Red that Horst drove was almost a copy of that car but, with Navi and CD Changer. This red one was about 16kg heavier then Stippler test mule... One other interesting thing is that both Stippler test mule and Horst test R8 was not equiped with Magnetic Ride... Even instructor R8s on Audi Driving experience are NOT equiped with Magnetic Ride... Interesting...
Regarding 7.29min... Well, Horst did 7.50min with grey GT-R test mule on the partially damp Ring(he did ONLY one lap... Karussel corner was completely damp)... I saw test data and sector time actually looked very good... Just few remarks-Doettinger Hoehe top speed was not that good and if Ring was dry Horst time would be in 7.45min-7.43min area...