take this FWIW, but when I first got my FRS, I had problems with hydroplaning on the highway during heavy downpours- which in Tampa is every day during the summer. I had a few oh shit moments in this car, I never had in my GenPoop. Just eyeballing the FRS, I noticed that it is built for being streamlined on the highway and lacks any sort of aero devices to reduce aero lift. The Gen has a small lip, small canards and is a lot wider than the FRS. It also was a lot more stable at speed.
So I added a front lip, side skirts and rear spats and BRZ spoiler. Soon after I added the gurney flap. Since then (even on the OEM tires) I have never hydroplaned. I even tried to once at a spot on the highway I knew where it happened every time it rained. Nope, I just plowed through the rivulet.
So does it make a difference? My whole aero package has made a big difference- no hydroplaning, very solid and planted at very high speeds and also, my MPG has suffered too.
Will someone soon post and say that it's all in my head, akin to a butt dyno, no way proven aero technologies can work on a road car? ... yup. Do I know my car and can I tell the difference at 70 MPH? ...yup