Keep in mind that at AC, ground and power planes are equivalent. If you have any high speed signals on the bottom their return current will travel in the "power" plane, not the "ground" plane. If those signals pass over a cut in the power plane, they will see a large inductance due to the return path disruption. This will cause stray coupling, noise, and signal reflections.
The normal reason to split power planes is because you have multiple supplies. If one region of your PCB is running on 3.3 V and another on 2.5 V, you would divide the power plane and try to avoid having any signals run over the gap. Since you are doing this because you are running out of routing space, chances are you will have signals crossing the gap. Try to keep that to low-speed, non critical signals and add supply stitching capacitors across the gap to allow a path for return currents.