I think the extra power consumption by IBIS will be more than offset by a newer, more energy-efficient PROCESSOR. Think of the newly released 1DX Mk III, it has the same size battery - exactly as the predecessor but is double the capacity. Further, as I understand it, the processor is not only much quicker (unlimited buffer, basically) but much more efficient as well. So I will not be surprised if the battery life of the R5 will not be somewhere between 140-160% improvement over the current R, perhaps even as much as 200%. I don't think it will be quite that much however. You still have battery suck from the EVF, the extra communication between lens and body, and as you mention, IBIS. But I do think a my initial guess is reasonable.