Hightower, i think that all RIBs are a compromise and the characteristics which are required to make a boat hugely quick in the calm and exceptionally seaworthy when it kicks up are at loggerheads.
For speed etc, you are going to need a different type of hull form. Long and thin tends to be the norm and a high power to weight ratio is a necessity.
However, the opposite is needed for an exceptional sea boat. A deep V with a very full bow and much chunkier construction is required.
Subsequently, the chunkier design will add weight to the boat which then results in loss of speed and so the circle continues to spiral.
By using different, lighter construction materials you may be able to get the more seaworthy design to weigh in similar to that of a standard 'quick' RIB but also remember that there is no substitute for weight in the rough as it will aid in keeping the boat stuck to the water and punching on through so the endevouring to loose weight may be detrimental to the boats capabilities.
So, youve got a heavy seaworthy hull which needs more power to get it to go quicker in the calm but the result is again more weight so top speed will, i guess reach a point when adding more power will be ineffective due to weight increase.
Again, i guess a compromise must be reached.
Remember also that the boats which tend to have the superior seakeeping characteristics are commercial and will nearly always have a heavier lay up, more heavy duty fittings and often heavy duty tubes, all of which add additional weight to the design and will lower top speed.
If I had to buy a 5.5m boat, it would be a searider. There’s no substitute for feeling safe and knowing that the boat will give up and fail a long time after you will!