The Laing DDC series of pumps (aka Swiftech MCP350 and MCP355) are great units, but their stock housing really slows them down, especially on the inlet. The aftermarket tops open the pump up and let them work far more efficiently, improving the pressure AND the flow rates right across the board. They also let you choose what size of barbs to use, rather than be stuck with 3/8" sizes.
Right now, XSPC's tops seem to have the edge, but it's not by a huge margin. Pretty much all the tops show a noticeable improvement over the stock configuration.
i7 2600K | ASUS Maximus IV GENE-Z | 580GTX | Corsair DDR3-2133