Good post. That helps us a lot to understand your point of view.
Any of the current crop of ST bikes is a pretty healthy step up on power from the C-10, but you know that already. I'm 265/5'11" and have 108k on my '04 FJR. I ride all year long, as you do. One of the things I like about the FJR is the available power in stock form. I average 44-46 mpg, but am not afraid to flog the bike when I want to.
Think of passing on two lane back roads. At say 50 mph, you have three easy options for the pass -
1. Just twist the grip and roll on by the car in front of you w/o any problems. B)
2. Down shift to 4th and twist the throttle and you shoot by the car before they can react. Good for those short passing areas.
3. Downshift twice to 3rd, whack the throttle. Time and Space fold together and suddenly you are 10 miles down the road and can't recall the last ten miles.
There is a lot of room for fun in the FJR and it's going to take you a little time to adjust to what's on tap, coming from the C-10. You should adjust smoothly and w/o problems, but it's natural to have a couple of 'Holy Sh&*' moments before you realize just how much
more power is on tap. I accidently lofted the front wheel the first week I was riding the FJR. (Light turned yellow, I whacked the throttle and pulled the front wheel off the ground all the way across the intersection. Much to my surprise, and never you mind the talk with the nice LEO...)
I hear what you're saying about making changes, Re: the Mustang. BTDT with a 300 rwhp turbo Miata. You
may be surprised to discover that the FJR does very well as is, or with relatively minor changes.
Bottom line, the FJR will do everything you like to do with the C-10 better, easier, and with you in more comfort. And you'll have this silly grin inside your helmet for quite a while too. The C-14 is a nice bike too, and has a lot going for it, but it didn't speak to me. You have to decide what fits you and your needs best. The silly little tank on the C-14 was a disappointment to me, all the more considering the C-10s big tank. It's a hot bike too.
Back on topic - Yes, there are mods you can make to add power. Generally at the expense of MPG, sometimes significantly so. But you're limited to probably 30 hp in total gains. While the FJR can be ridden high rpms, and most of us do in the twisties, it doesn't need to be wrung out to have a good dose of toque and motivate down the road well.
If you want a replacement for the C-10 that has a known upgrade path, HP wise, then the FJR is probably not the best choice. If you want a replacement for the C-10 that is an all around upgrade and has the potential to keep you happy for a while, the FJR can do that.
Hope that helps