Maybe I'm missing something here but since they recommend that the steerer plug be located at a certain place in relation to the stem why not just move the plug and add whatever size spacers needed on top of the stem. There is also the option of taking the extra 2-3mm off the steerer with a sanding wheel preferably the motorized on a stand type for accuracy as opposed to a hand held drill. (protect your lungs and wear a mask) I always prefer to have a little extra stem on top for two reasons. First you'll have more adjustment available if you decide to raise your stem at some point and second you give just a little more clamping area for the stem. That 3mm that sits halfway between one of the clamps creates a higher stress spot. Probably not a problem for most setups but why do it that way when a spacer on top takes away the risk.