[Flojo]

absolutely your lowering and slight rake now made the difference. simple aerodynamic logic.

FYI: Porsche actually prohibits to use the "turbo" lip without a tale (either duck or whale) for aerodinamic reasons (and vis versa).
the downforce must be balanced front to rear equally.

what is your "high speed"?

I do pusch my SC on the Autobahn around 200 km/h with just the front lip and no issue.
setup: lowered just below Euro ride hight, camber -1.4 front, -1 at rear, running 7/9x16 (205/245), with Bilstein B6 hard.

at 220km/h and above the car gets uncomfortable at rear as the front downforce begins to build a kind of leverage so the rear gets lifted.
but you can handle it