The acceleration between the two cars is close but the GTR simply gets to 60 miles per hour faster which gives it an edge in the 1/4 mile with an firstname.lastname@example.org versus the R8 email@example.com. The R8 is pulling slightly harder at the 1/4 mile mark but the GTR gets there first.
The disparity in handling surprisingly is greater than the disparity in acceleration. The R8 is an excellent handling car posting a .99g skidpad number but the GTR pulls a 1.04g. On the MotorTrend figure eight the GTR again bests the R8 with a 23.0 @ .93g versus 23.5 @ .90g. Skidpad and figure eight numbers are decisively in the GTR Track Pack's corner.
This translates on the track where at the Streets of Willow Springs the GTR laptime is over 2 seconds faster than the R8. 2 seconds on that tight course? Despite a 200 pounds weight penalty? And the R8 being quicker around the track than a 458 Italia? Wow.
Full results are below. The R8 V10 Plus is incredible and an improvement but the GTR certainly is amazing especially considering the value versus the R8.