Thanks for all your Nacra info.
and frankly the 10' beam is overkill (it just means you'll be in the trapeze in 12-13kts of breeze instead of 10 kts as an example).
But I cannot completely follow you here.
If you are sailing on flat water with a 10 feet platform, you can have more fun by taking for instance your grandchildren (or even grandgrand children) with you.
The more surface on your platform, the more social opportunities.
If you are sailing in waves at sea, in my experience, you feel every inch of more beamlength. I cannot explain this here theoretically, but with a labile equilibrium it comes to the millimetre.
I discussed that once with the Dutch Nacra people in Scheveningen and they totally agree. Thats also the reason why in the design of the F17 they broke with the 2.5 meter barrier, the famous legal roadregulation.
So, also here counts the more inch in the beam, the more fun. I wouldn'd call it an overkill.
Downside however is, you cannot tack anymore when it is too much!