I also stopped wearing Sidis because they were too narrow in the toebox for my feet. If the difference is only 2mm I'd be inclined to go up a whole size. But that's partially based on my experience during long summer rides when my feet tend to swell, so I wear fairly loose shoes to account for that.

Is there any way you could go back to the LBS to measure different sizes of the shoes they have there? Either that or contact Sidi directly and ask them for more information on half sizing.