I used a roof rack on my 16V and now on my Williams too. I need to transport my bike around and its far better when you need the space for other people/luggage.
Even without a bike on top, it will seriously blunt the performance as the speed rises. I remember having trouble shaking off an E46 BMW 318i with the bike on top of my Valver - normally, Id have been chasing E36 328s.
I have a Thule system and its well made and fits the bill. It cost about £180 all in at the time with a bike rack included.