Hi Wouter,
I'm not sure what 'just right' is. It is not a nautical/boat design term that I am familiar with. I don't follow what it means unless it means 2:1 boat length to beam ratio. There is no magic ratio or perfect ratio or just right ratio that I know of.
If a Tornado was 12ft wide, it would be the same speed/performance as today's Tornado in light and medium winds. When the winds go higher than 15knots, the wide Tornado would be faster to windward, faster on a close reach, faster on a beam reach and the same speed downwind.
Any time there is enough wind to utilize the additional 20% in righting moment, the 12ft wide Tornado will be faster.
Now, there are some conditions where the over use of righting moment can drive a boat to pitchpole. This is no different whether the boat is 10ft wide or 12ft wide or 20ft wide. You still have to watch the leeward bow and make your decision. The sailors operating the boat have to make the call as to how much righting moment to apply. Righting moment to a sailboat is just like horsepower to a race car. The operator has to make the decision as to how much power to apply. In a highly powered race car sometimes the driver has to back off the throttle to maintain control. So it is with a high powered catamaran sailboat. Sometimes you have to get out of the throttle a little to maintain stability. That's part of the skill and fun. Monohull sailors don't know what we are talking about.
Bill