I wouldn't trust ebay for discs. But that's your decision.
I had Rossini discs on my roadster, they stopped just fine. They're not listed on the menus on their site but they do definitely do them. Just ring them.
http://www.carparts-direct.co.uk/home ... i-performance-brake-discsAbout the braking, it took me a very long time before I experienced Braking Assistance on the roadster, this is an emergency thing where the roadster sees you're stopping VERY hard and takes over to stop the quickest possible! You feel the brake pedal drop further than you could push it from under your foot and boy did it stop! If I recall rightly it even puts the hazards on. I experienced it on the motorway at 70mph, I didn't like it as it actually stopped in a shorter distance than I would have aimed for! I remember hoping the car behind could stop as quick, luckily it was well spaced, as was I, so I really didn't need it but got it anyway!