Some mis concept issues:
My 380 sib with 18 tootsie only weights 120 KG, tested on extreme nice water conditions, 2 mates weighted 120 Kg (60 each) 10 kilos for 3 gal gasoline making 250 Kg total The test was made in a rowing field 1.5 Km long at sea, the drivers were jockey sitting inside middle deck, seems speed is increased a bit over sitting on top tubes and also betters lower center of gravity. Sibbing alone at wot with same conditions get close to 40 Km/Hr and I'm not a feather champ.
Ribs are much faster than sibs, other issue when speeding in a sib or rib you will have at least 1/3 sib out of water, much less drag to move. Running at 30 or 40 Km/Hr over things might get you into trouble, so sib safe. Usually running at 3/4 throttle, that is 30 Km/Hr as don't work for a oil Company.
Best final speed is directly related to water conditions, wind, currents, sib weight, balance and proper engine tuneup & mount. My final bet would be around 55 Km/Hr in best opt conditions.