You'll be amazed by how much power you loose without one.
If your thinking about getting one then don't.... just do it - SeatCupra.net

Totally silent
Car doesn't squirm at all now when pulling away with full WOT. Its dead straight and feels fully planted. Can't feel any power loss either so TC mustn't be grafting too much now.
Car feels a lot more planted on the road and no longer tramlines or hunts the grooves in the road. Generally feels less 'floaty' at speed.
Generally feels better - Piston Heads