I wish I could tell you the effects of a fully covered underbody at stock height since my new BRZ is now on stock suspension while I wait for the back-ordered Bilstein B6 to arrive which will be paired with RCE Yellows (0.8" drop). However, I haven't had a chance to drive it beyond 120 kph yet.
However, when my FRS had the underbody fully covered (Beatrush skid plate, JDM panel covers, Verus trans tunnel cover, Verus diff/susp cover, & Verus diffuser) with TRD rear spoiler & Bilstein B6 + TRD lowering springs (1.125" drop) with 18x7.5 +225/40 wheels & tires, the car was more planted especially in the train even at 80kph. When I ran it at 180 kph on a dry highway, it was even more planted & stable & you know that you can push it for more.
At stock height, you won't scrape unless it's really high or steep. At 1.125" drop, the rear diffuser will hit the parking lot's wheel stops while the underbody will scrape a steeper ramp or even a higher speed bump a little especially if you have passengers. Mine had scraped several times before but there was no damage on the covers.
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Modding for daily fun & efficiency: Gruppe-S UEL, HKS dual res. FP, STi Per. Muffler, Delicious T. Ecutek tune, Crawford BPB, Grimmspeed CAI, STi Flex T. Bar, TRD Door Stabilizer, DSS aluminum propeller shaft, Verus diffuser & underbody covers, Verus LCA, Perrin bushings, Bilstein B6+RCE Yellows, Michelin PS4S + 18" WedsSport TC105X-summer, Michelin Alpin PA4 +18" Motegi MR140-winter