Mine's flat front to rear, with the rear undertray bolted to the standard marlin lower valence. I don't think it's technically correct to call it a diffuser, as the angles don't match with what you'd expect.
The rear of my car is slightly elevated by about 10mm.
The fastest I've been in the car on track, is about 125mph on GPS.
My wing is slightly larger than Ken's, but mounted lower, and I did have issues with the bodywork flapping at speed originally, but solved that by welding a bodywork mount to the middle of the rear bulkead, and securing the shell with a Sparco bonnet fastener.