Visualize each pump out on your lawn side by side with only the inlet connected
Turn each pump on and water will shoot straight up in a vertical column.
The mcp655 will shoot 11 feet in the air at zero psi where the column from the mcp355 will be 20 feet at zero psi.
Although psi and gallons per minute are is part of the equation the efficiency of a pump is actually measured in flow rate or head of pressure.
The difference between the two pumps will not be noted until the loop reaches its highest point of restriction.
With multiple high end water blocks and large radiators the mcp355 with the modded top would be the better choice.
Even with this information the mcp655 is no slouch and a very good pump except it seems to dump heat into the loop on its highest setting (adjustable models)
The only thing I would question is at what rating will the mcp355 be with the larger barbs?
May be when I get a lawn I’ll try this