Highline computers offer you more functionality than a midline computer. If you have a midline computer you get basic features such as average mpg, miles to empty, average speed, journey time.
A highline computer offers the same features as midline plus additional features such as display for sat nav, track names for ipod/mp3, and full integration of phone book if you choose for the optional bluetooth phone prep. There are also different settings for your car you can access through the highline computer. Highlines are standard on GT models and a fairly low cost option on base model cars. Well worth having though in my view.
I don't have a Scirocco myself and don't know what you've read, but I suppose from a purists point of view the idea of a diesel engine in a coupe is a little difficult for some to stomach. I have a 2.0 diesel Golf and in my own humble opinion the blend between performance and economy/running costs is excellent. If performance is what you're after though, a TSi will undoubtedly satisfy you more. The added weight of a diesel engine upfront may blunt the sharpness of the handling/steering, although I think you'll probably find the difference is slight.
Not sure about the headlining - I could be wrong but it may be that it's light grey for standard models and dark headlining for GT spec cars. Like you say the brochure doesn't make any obvious mention of this.