I find the overall balance to be quite similar, structurally. What I am referring specifically to is the rolled off upper register that the HD650 has. I think the HD650 is relatively untextured, but it's much easier to discover new detail in it compared to the P5.
I actually find the P5 to carry a lot of texture and detail throughout the register, but not in an easily accessible way; this is more what my comparison to it as a sort of "HP1" style sound entails. The P5 is a bit slow and plodding at times and excels primarily with vocal stuff where its ability to resolve sounds does an excellent job in making voices sound natural.