I had heard the Gnus ride big, so am tempted to drop down to a 156.
It sounds very much like you are trying to find the correct cm size that you would ride in any of these boards. I would highly suggest that you abandon that goal. At 150 lbs, size 9, 156 will be a much better choice for you in the Billy goat than the 159. In the Impossible you will want the 152.
The tip to tip length of a board is not a valuable measurement. Advice like, "Gnus ride big" is equally incorrect. Every board model has been designed for a specific rider weight range and foot size range. This will vary from model to model even within a brand. The reason your 159 was feeling unwieldy was almost certainly because it was too large for you in that specific model.
Once you have this corrected you will be very pleasantly surprised by how easy the correct board can be!