With excess weight aloft in a boat will markedly decrease its stability. One New Years eve, I (sober) took about a dozen of my friends out on our Symbol 42. Heavy boat with 500 gallons of fuel, 300 gallons of water and two Cats down low, and a little trim ballast in the bilge. 6 were on the flying bridge, and 3 in the main saloon. Three who had a bit to drink decided that they would run back and forth across the foredeck (flush--no cabin) and see how much they could roll the boat. After a couple of trips back and forth, I put an end to that. I don't know if that 600 lbs shifting back and forth would have capsized the boat or not, but it sure got it rolling a lot more than most seas we encountered.
The 34 Silverton had some rough weather, plus a wake according to reports. Just those factors, with an overload would be enough to expect that stability would be minimal.