Pump top is an optional accessory that give you more fittings options and improved performance (higher flow rate/head pressure). The stock MCP655 does not take any compression fittings and the fittings on the pump is plastic, most people like something sturdy will get a top and use metal barbs or compression fittings.
Pump reservoir is again optional, does it give you better performance I don't know so. But pump reservoir is more flexible in the placement.
I have this
aftermarket for my MCP655. You can find other tops just make sure it say compatiable with D5/MCP655 pump.
If you are looking for reservoir to fit the pump this
reservoir for the MCP655 is a good choice, but I don't see the point doing so since you already own a reservoir.