You would have to take 15†off of the foot of the main, not the top. Truncating the top would result in a large roach area that would get hung up on the backstay every time you tacked. The reef points would also have to be moved up, and a new clew created. That is a lot of sailwork.
A better alternative might be taking enough off of the leech edge to result in a 15†reduction at the head. This avoids moving the reef points, but requires creating both a new head AND clew. Still a fair bit of sailwork.
In any event, the mainsheet would have to be lengthened by 60†(15†x 4). And the vang (if you have one) lengthened as well.
This mod would raise the center of effort of the sail, resulting in more heeling, and thus more weather helm, at a given wind speed.
Might be simpler to just lower the height of the Dodger!