It would be an upgrade, but the most common reason for 5400RPM note book drives is to combat heat and power consumption. With a fast, larger drive you may find yourself loosing more battery time and generated heat that could self destructive to the drive. There is only a small chance of that, but something to consider.
Yes your Laptop drive cab be shared out through an external enclosure. Then best part is you can get one with passive cooling powered over USB2. Check the usual sites or stored for them.