I'm far from an expert on the 400, but I thought the factory tire WAS a 110/70-17? I don't know of anyone who makes a 110/60-17, but I haven't looked much. You should be able to drop the fork clamps down the fork tubes to lower the front, however.
You can likely use the 150/60 on the rear, it should lower the back end approx 1.5 cm.
As long as you drop the front and rear a similar amount, you shouldn't notice a big change, but the shorter rear tire may slow steering slightly, since it IS a little "flatter".