The Westin SPC-1100/2 twin fan extractor was chosen after consultation with Westin's technical department. It extracts at up to 2000 cubic metres per hour. Extractors in mantels tend to be less efficient as they don't cover the front burners of the hob that well and consequently the extractor has to bend the air from this area. Twin fans help that process.
Westin also make a range of custom build extractors. In this kitchen they made the stainless steel splashback and bottom cover for the wall cabinets with a cutout to house the extractor as seen in the picture below.