Thinking of upgrading my brakes to these, is there an issue with "mushy" braking? I want to get these in combination with new levers for my bike, but under the description it says:

"Use of these calipers with standard SLR enabled STI and brake levers results in a "mushy" brake response"

I've got a simple, single-speed bike, I'm not sure how this will be an issue? All of the reviews seem great, and they're the only reasonably priced shimano brake calipers that are black to match my gear.

Maybe I'm missing the mark completely and these brakes aren't even meant for a bike like mine, if that's the case can anyone recommend some affordable brake calipers that I can upgrade to?
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.