There are a few other advantages to the spey rod. The greates of these is the time your fly spends in the water. There is no arguing that the more time your fly spends in the water the greater your odds are. This is especially true when swinging flies. I have a 10' Sage XP that is a great rod for delivering wets, and I consider myself fairly adept at casting it. Before I really got into the two handed gig I fished it for a week on the Umpqua blasting out casts in the 70-80 ft range all day. I have also fishe 40-50 wide streams in upstate NY with it.
What I can tell you is that I fish both those situations with a spey rod now. The small streams I cover with an 11' 8 weight with a windcutter. I don't need a spey for these streams but my fly spends probably 25% more time in the water actually fishing when I have my little spey rod. No roll casting, no false casting. Just two quick flicks of the wrist and my fly is back in the drink doing its thing. Although often not needed, two handed rods are almost always more efficent than a single handed rod, even at close range.
As Skagit lines start to come out on the market I think they are going to be recognized as a great way to efficently cover closer lies with a deeply sunken fly on shorter spey rods on the great lakes.
The added power in the butts of these rods allows a sink tip or heavily weighted fly to be lifted and immediately cast without difficulty.
As far as the fight with a spey, it's actually quite the opposite of what your thinking. Although the butts are large and powerful the leverage advantage the fish gains more than makes up for it. For example, I have fished 10 weight spey rods that a 12 lb steelhead feels great on. On the other hand I have a 9' 10 weight and it would be total overkill for the same fish.
I encourage you to try a spey, I think you'll get hooked. If not just keep plugging away with the single hander, I doubt anyone will hold it against you