Here's a good tool to compare boats: http://eddyflower.com/BoatSearch.aspx
I'd be a little concerned b/c the Siren is only 40 gallons where your EZG is 50. The EZG 42 has a recommended paddler weight of 80-140 (the EZG 50 is 130-190). Might be a big jump in the way it performs going down river. I'd think the weight range for the Siren would be less than that of the EZG 42. Even though you can roll it better, you might be getting flipped over more.
I'm just a touch heavier than you and my smallest boat (Kingpin 6.2, playboat, not a river runner) was 47 gallons. Personally, I wouldn't go any smaller. A 2Fun at 46/47 (classic/new) gallons might be something good to try.
I over heard a friend talking about boat volume once when someone was dismissing a 5 gallon difference. He said, try to push a 5 gallon bucket under water... got me thinking. Volume, design all play key rolls. Do a little analysis with the page above. It will get you a little more familiar with the differences between models and sizes.
And, I don't think I recommended the Happy Seat with enough emphasis before... I think it would really help with the fit of any boat.