Dear MOP, yes I do have the same question too .. more even since my weight is 72-75 kg [ no clothes] with a wetsuit ..we are a lot heavier of course not to mention boom/mast/sail.
Is a ULTRASonic no overkill for me .. r would it just work .. is an ultrasonic not only for heavier riders .. do you need as many liters for 75 kg surfer to plane as quick as possible or is the width that makes the biggest difference?
WOuld an IS 127 not better for me. I currently sail a FAnatic ray 145 ltd  with a 9m Maui Sails Titan and it gets big quickly. I just want to plane earlier and have more control in the gusts.
I know formula is the king .. but I wanna have lot's of fun to and a board that serves me well for years to come.