I wouldn't focus too much on grooves for rain performance... in the end, it seems that a soft compound is at least as important.
Here is an example of driving relatively well in the rain: Oct 6 2013 BSCC Rain Autocross - My Fastest Run - 2012 V6 Mustang - YouTube
As you can see, you can't give a car much throttle in the rain if you want the tires to stay hooked up.
That run was 6 seconds faster than the next 2011+ mustang that day and there were at least 6 of us. The difference was mostly that the other guys were using way too much throttle and going into slide after slide.
2012 V6 Base. 6-speed. 3.31. Roush Wheel Hop Kit. BMR 38mm Front Swaybar. Camber Modded Koni Yellow Struts. Linear Steeda Sport Springs. J&M Camber Plates. J&M Panhard Bar. Airaid Intake. 93R Bama Tune. BBK Ceramic Shorties. Carbon Fiber Trac-Lok w/ Finned Cover. Hawk SR pads. GT Rotors. Spec Al Flywheel & Stage I Clutch, 18x10.5 ET38 Enkei PF01 Wheels. 285 30 R18 RE-71r tires. Boss 302 Radiator.