Picker,
Just wondering if you've tried other string materials before moving up to mediums... I.e., if you're looking for more warmth, have you tried Phosphor Bronze and or Red Brand Copper strings? If you're looking for more edge, have you tried 80/20 Bronze? If you're looking for a mellower, quieter sound, have you tried silk and steel?
Just keep in mind that there's about 25 pounds more pull on the bridge/neck with 13s than with 12s. You might consider tuning down a half step which would mitigate that. You also might find that some of the wound strings bind a little bit in the nut during tuning. Use a pencil lead and grind a little graphite into the nut before you string 'em up.