I suspect the perceived effect of the lift is greater at the front simply because the front wheels are largely responsible for providing feedback when making minor steering corrections. The OEM springs led to a wandering front end at speeds over 120-130mph, largely rectified by fitting Eibachs and absolutely resolved with my current setup. I've not driven a 3.0 Coupe so I don't know if they have the same handling characteristic at very high speed - do they have unusual spring rates as found with the OEM ///M suspension? How much rake is there with the OEM setup in either car vs Eibachs - perhaps this has some bearing? Perhaps the front suspension permits more travel than the rear in response to any lift created by the car, thus leading to a negative rake which exacerbates the front end lift. It's even possible that the vague front end at speed in OEM guise is not related to aerodynamic lift.
Dan Gurney himself found that a Gurney flap in isolation can increase rear down force to such an extent that the front of the car will lift leading to understeer and slower lap times. He responded to this by increasing down force at the front. With this combination, he was able to achieve higher straight line speeds as well as higher cornering speeds. :
