I am a former CAT 3 roadie that converted to tri/ du. From a cycling standpoint, CLipless pedals offer a variety of benefits over using flat pedals and running shoes on the bike. As many have said, the inherent benefit is the ability to utilize the full pedal stroke, rather then just a down stroke. By full pedal stroke, I refer to the ability of a cyclist to not only push down, but push up on the pedals through the full range of motion of the cranks.
Clipless pedals really allow one to work on cadence, by pedaling in "circles" rather then "mashing" the pedals down. This is a drill that most competitve cyclists use in the winter on their trainers, to improve cadence. Move the pedals in a circle, and see how much more efficient you become. You should see this refelected on your cyclocomputer if it has the ability to read cadence.
When shopping for shoes, remember that pedals come in a variety of flavors. The tradition rad pedal is whats refered to as the "Look" style, which is a wide cleat that clips into a one- sided pedal. Many road cyclists use mountain bike pedals, which are generally double-sided and offer more "float" which is a term that referes to the amount of lateral
(side to side
) movement your foot has while clipped in. This is very important, because the dynamics of the pedal stroke tends to move the foot laterally depending on the pedal stroke. People with bad knees often complain that pedals with little float aggravates their knee pain.
Also, look for a shoes with a stiff sole. Many of the upper end
(i.e. expensive
) shoes have carbon fiber soles that are extremely stiff. Stiffness is important, because you dont want the shoe to flex in the pedal stroke- you want to apply the maximum amount of power to the pedals.
Any questions email me offline if you like. You may not see any instaneous results by going to a clipless pedal system, becuase it will take some time for you to adapt to a more efficient circular pedal stroke, but you will eventually.