Cars do not need power steering, however in a parking lot it makes maneuvering allot easier. But once they are moving the power steering is not really needed. It is similar with motorcycles and with repect to size and maneuverability, size and weight, smaller and lighter is easier to move around, however at speed the maneuverability is a matter of the geometry of the design, a larger bike can be very maneuverable and responsive.
As for power it should be smooth and linear, from what I have read the new generation of 250-300cc bikes have that. I believe the new R3 has that, so does the CBR300. However as you move up into and above 500cc prices go up with that and so does insurance.
The thing is that all bikes have a power band, often many ridders stay under it, so if it is a 300cc or a 500cc up to that is not much difference, once in it the differences become apparent and in some cases not as different as you would expect.
The point of saying a 150cc is enough, would be a person that is not interested in the power, just the experience of being on two wheels. The 150 can get up to speed, it just that there is no power band, lots of screaming to top speed but nothing that would be considered a burst of power.