The aero issue in GT6 also lends a hand on how the PP is faulty, imho. Testing cars lap time against it's PP on high speed track like Midfield will only make the aero issue even more pronounced. Above 180kmh, most cars with no aero will have big advantage in acceleration and top speed, while cars with high base aero ( either fixed or adjustable ) will need to compensate in cornering. Tracks with more straight portion and high speed will favor cars with no aero but still have good power/weight ratio and mechanical grip. The PP system in GT6 disregard the aero, unlike in GT5 ( which have better aero drag than GT6 ).
A simple test, get the Amuse/Opera 350Z RS ( it has aero ) and Amuse 380RS SuperLeggera, drive them on high speed track and notice how both cars accelerate above 180kmh, now get the 350Z 2007, set the power and weight similar to both cars and do the same run, and notice how it accelerate at high speed, also make a note of how much PP the 350Z 2007 has ( with closest power/torque/weight )
PD needs to overhaul the PP system, I recently compared my R34 GTR replica build on both GT6 and GT5 ( ver 1.0 ) and the aero issue immediately apparent at high speed track. Tested R34 GTR in GT5 at La Sarthe 2009 no chicane for top speed and acceleration. The R34 GTR struggle above 180kmh and barely went over 270kmh top speed and took a long time to get there, and GT6 .... the R34 GTR pulls strongly above 200kmh, easily reach more than 10kmh higher top speed ( 280+ ) earlier/quicker. Both on same CS tire, same power/torque, same suspension/lsd/gear ratio, same weight and aero ( GT5 uses smaller number, GT6 multiplied GT5 value by 10 ) GT5 aero at 5 is similar to GT6 at 50.
Tsukuba would be better track for comparing cars as most road cars will not top around 160-180+ kmh at the long straight.