I'd definitely advise against trying to convert a pair of shoes. If you're going clipless for high efficiency, you really need the stiff sole of a proper cycling shoe. Yeah, most of them look disco-themed, but they're a whole different beast from a pair of modified tennis shoes. Plus, tracking down cycling shoes in your size sounds like a lot less hassle than converting other shoes. If you're not particular about the super-duper power transfer, regular shoes with clips or powergrips sound like a better solution. No need to suffer walking around on stiff soles if you don't feel you need them on the ride.