The object in making the sandwich is to reduce the weight of the top. Since most of the stiffness comes from the surface layers, you can use almost anything in the middle, so long as it will glue well, hold up to the forces involved, and is mostly air to keep the weight down. End-grain balsa has been used in this sort of structure for a long time, and works well. The main issue with it may be that it weighs more that honeycomb, so you don't get as much weight reduction. One might expect that the balsa core would not be as loud as a top with Nomex, but could sound a bit more 'traditional'. AS has been said, and as always with guitars, you can't say much for sure without the instrument in your lap.