I spend a lot of time on the mountain roads up here in New Hampshire and Vermont and tour a lot up into Canada (Nova Scotia, the Gaspe, the Cabot Trail, etc). I now ride a 2014 RT Limited, but I spent close to 50 years running those same roads on an assortment of BMW's so it's something I'm used to. Let me ask you a question. Are you a "hang off the bike" kind of rider or a "keep your butt planted in the seat" type of rider? If the former, then you will find the Spyder lacking for this type of riding. There is no way a Spyder RT can keep up with a talented rider, on a good bike, thru the twisties. I don't know about an RS or an ST because I've never ridden them, but an RT is a 1000 lb bike and that has an impact on how well it corners.
However, having said that, with the right technique and the proper suspension setup, the Spyder RT will do amazingly well. Once you master it and get it set up right, I would say you can keep up with about 80% of the riders out there on those types of roads. Start with a sway bar change (I chose Baja Ron's) then move on to Elka shocks and decent tires and you should start to get there. Let's face it, it's all the things you'd have to do to a decent car to get it to run those same kind of roads.
The rest is technique. Start slow and learn how to lean into the turns, how to brace and put your weight on the outside leg, and learn the feel of the Spyder thru the corners. I would not say that the mountains are the RT's forte, but for a touring machine it does pretty well considering you can't lean it. And what it lacks in these areas, it more than makes up for as a general touring machine.
Come back and ask me at the end of the summer and I'll tell you how well it kept up with all my buddies who are still riding Wings, BMW's, and Duc's. If I can keep up with them on the RT for general touring thru the mountains and up into Nova Scotia on the Cabot Trail, then I'll say that it's doing just fine.