I'll repeat what I read earlier here. I think it was in reference to the Hot Rod Magazine LSR 2nd gen Camaro. Lower, lower, lower. The lower you go, the better off you'll be. There's some debate on what is "too low" (about five inches, the ground clearance on Ren), but from what I gather, you're unlikely to see any gains by increasing ride height. Even SUV's like the Touareg with air suspension lower it bodily on the freeway. I think the Ram does this too now.
PS- food for thought. Companies in Europe probably make air kits for your Polo, and also probably for the E36 in your profile photo (although that 318 may be a mix-n-max with an E30 kit). You could conceivably convert to air suspension for fuel economy, and though the gains could be significant (10% maybe?), you may not make back the money spent.