first of all thanks for your clear reply.
As I am a 2008 Vapor user I'd like to share my opinion with you.
This year is first time in big slalom stuff as I used sail fw in light-med wind.So I've had to learn a couple of things.As big slalom board I own a Manta 79.I feel that the 9,0 is big enough for my 80 kg in true 11 knt.But if you sail a Starboard 133 in the same wind probably you need more pull from a slightly bigger area.That look a nice reason for asking for more mq.
When it comes to PWA the point is a bit different.I've heard that you all like to start racing when steady 13-14 knt.Oh weel.it can be different if during the event the wind lacks.In that case every chance to race will be run.As I'm not a PWA rider I can be wrong.Anyway.That's could be the other reason why you want the 8,4 instead the 8,2.It still can have that "grunt" to pull you out of trouble if needs but it's still the correct boom lenght to be smooth in manouvres.It is going to be the smallest "efficient" size for the 133 or the right size for the 122.
As for Vapor's range I find it great both for the 12, the 9,0 and 6,7 while I'm pretty disappointed about the 11.When overpowered if you OH a bit more than specs the sail looses speed and angle while mantaining the truck pull that tires you off.Right now no way to make it work right.
Please note that I use to set the pulley between specs and + 2/3 cm in every size with Gaastra Gold mast.