I've just tested this out in Arcade mode.
Polyphony Formula GT at Suzuka with weather change time trial. Default settings. Rain Tires are harder to use to produce a quick lap time. Putting Racing Softs on makes it easier provided you are easy on the throttle coming out of corners and through fast corners.
I was disappointed with this result, so I wondered if it was just an anomaly with the Formula GT. I had been doing hot laps earlier with a Zonda C12 '00 at Monza, so I knew the track and speeds I was going in the dry, I thought I'd give it a go in the wet.
Default settings again, I took to the track with Racing Wets. Slower lap time than in the dry, reasonably difficult to control. Put Racing Softs on, and it was quicker down the first straight and out of the first hairpin. Appeared more difficult to control through the long fast right hander but other than that a better/easier performer.
I went back to Time Trail menu and selected Monza Change weather again and the Zonda again. This time when the 'Driving Options' dialog box comes up, I went right to the bottom and changed the option of 'Grip Reduction on Wet Track/Track Edge' from Low (the default setting) to Real.
Taking the Racing Wets out again, it was about the same difficulty as before. Maybe a little harder to control but not far from what I was experiencing with the Wets in the previous test. So I bang on the Racing Slicks......I could barely make it around the Parabolica to begin my first lap. In the dry, you drop down to about 135kmph to apex the corner. With the Racing Wets on it was about 110kmph, maybe slower. With the Racing Softs on in the wet, I was having trouble at 70kmph! I gave up before I crossed the line for the first lap....
So I guess in GT Life, the default settings for the races is set to Low Grip levels, as opposed to Real, hence why you can get away with running Racing Softs in the Rain in GT Life.