I don't think capture speed has any effect on the sharpness of the image, sharpness is more down to the lens/shutter speed, I'm assuming the "speed" touted by Apple refers to the startup time of the camera app as well as the responsiveness of the UI and the ability to take consecutive shots faster.
Looking at the shots again, it does seem that the sharpest image, that of the original iPhone 4, is also the noisiest, iPhone4 autoexposure set it to high-iso/fast shutter I suppose?