I use a 3.4m hypalon Sport, current model.
Not sure of precise speed, haven't checked it on a GPS. However, my guess would be over 25knts at full pace with one person on board. I generally have two up, and occasionally 3, which isn't comfortable with all the dive gear.
The 18hp is within the weight bracket suggested for the quicksilver, so that is why i use it. Apparently tohatsu outboards are just modified 15hp mercs as you stated.
The duck handles the waves fine, as mention it is so light and maneuverable that i generally avoid throwing passengers, and with the extra power can bail out of situations quickly.
Personally, i dont dive off the duck when the weather is poor, but have been caught out in changes of about 20-25kt SW, and a bit of swell. I felt very comfortable with the boat and at no stage thought I was in danger.
I noticed that you want to do rough weather play, personally i think RIB's are better for that.
I have a few mates with 3.8m Quicksliver duratex/Achilles, with 25hp. these are slighty less maneuverable, but still handle all conditions we dive in with ease.