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.