Going by the specs, the power gain is negligible, from 3.7hp to 3.75hp. Only way to detect a 0.05hp difference would be on a dyno. But being a strato, it should have a broader power band, which will make it feel like a greater boost in power than the specs indicate.
As for losing weight, we all know how reliable Husqvarna's weight specs are. At least it's likely that the 550 will not weight MORE than the 346; if they've exaggerated to similar degrees on both saws, then the 550 should weigh about 3 ounces less than the 346.