Modern short shank cornet mouthpieces won't (or at least shouldn't) affect the intonation of a cornet, at least not the way you want. The shank taper is the same as the longer cornet mouthpieces, so they won't go in any farther. Ok, but they're shorter, right, so don't they make the horn more sharp? Well, no, because they're short to counteract the way their large, deep cups and huge throats play flat. The only experience I've had with one altering the tuning of a cornet was to make the upper range a little more flat. Now, if you were to find a short shank mouthpiece with a "normal" size throat and cup, then the cornet might play more sharp, due only to the reduced length of the mouthpiece.