In the real world they very much are similar on most road courses. However, in my computing simulation process they appear to be 22 seconds apart on Germany's north loop both in standard form and 18 seconds apart in Weissach form
GT3 RS: ((20600m/(300km/h/(321m*0.3048m))x(1497kg/589hp)/1.31g)x0.0095 x 0.700 to 0.725
GT2 RS: ((20600m/(300km/h/(301m*0.3048m))x(1523kg/775hp)/1.31g)x0.0095 x 0.925 to 0.950
A Weissach-equipped GT3 RS is capable of producing a maximum of 841 hp on Germany's north loop circuit, while an equivalent contemporary GT2 RS is capable of "only" 837 hp. Sorry GT2 fans, but the GT3 is the real track star.