Thinking of upgrading my brakes to these, is there an issue with "mushy" braking?
It's counterintuitive but brakes with a mushy feel are actually going to have more power. This is due to the difference in cable pull ratio. It also means you'll have to run the pad a little closer to the rim, which can be an issue if your wheels are out of true.
Overall, I think this would be a great setup. The new 105s are great brakes, and you'll have plenty of stopping power.