Nikon SB-900 vs SB-800: Output Power and Recycle Times – WNPmedia:
It's not the output power they improved but the recycle time. If you use the fifth battery with the SB800, it will still not re-cycle as fast as the 900. They both overheat it's just the 900 makes it more obvious. When you kill the temp monitor on the 900 it has fractionally more output – probably the wider lamp – but it fires faster at full power… And then dies faster! It also costs more to repair.