On Lake Bala in a gusty F6 yesterday, BR20 Gladys was wearing her gunter rig with two reefs in, while Turaco had her reduced Swiss mainsail with one reef and later with none. Both boats were ballasted. With one reef in, Turaco was quite slow but unreefed and downwind, at least, the two boats were neck and neck nearly all the way back to the slipway. Both boats exceeded hull speed for significant lengths of time.
After 18 months of using the Swiss mainsail as a mizzen staysail, the running rigging wasn't quite right but can easily be improved. The sail was fast to rig, reef, unreef and derig. I can see the attraction of the full-sized BRe fathead mainsail. Track below.