Let's get this straight before I ramble on. Moving the cleat back, means moving it toward the heel, therefore having the feet more forward over the pedal...right?
A while ago I was doing some commuting on a cheap bike while wearing jogging shoes, and using toe-clips and straps. It just so happened that the toe-clips were extra large, and the ball of my foot was much further forward on the pedal.
I felt very comfortable in this position and felt as though I had more power, especially when out of the saddle, so I experimented with my old Adidas cycling shoes on my "good" bike. I even dug holes in the soles to allow the cleats to go way, way back, way past the manufacturers limit.
I did like it for a while, and, as with my commuting bike, I felt stronger, especially out of the saddle, but I had some difficulty getting comfortable in the seated position. I like to have my seat high, and it sometimes felt as though my knee was way behind the ball of my foot when the crank was at the 3 o'clock position -- like I was kicking my shin forward.
I've gradually moved the cleats slightly more forward, but still have them further back than most. I recently bought some Sidi shoes (size 10 US), and I have the cleats about 2mm from being all the way back.
So, in my opinion, having the cleats a fair way back is preferable, but the limits set by the manufacturer provide enough scope.