You've spent a lot of money on this car. Now go spend $39 dollars for a tool.

Go to Harbor Freight and buy an IR temp gun.
Get the car up to temperature.
Measure the temperature of the water neck.
Measure the temperature of the radiator where the upper hose goes in. It should be very close to the temp of the water neck, assuming the thermostat is open.
Measure the radiator where the lower hose goes out.
There should be a minimum of 30 degrees temperature drop in the radiator. If not, measure the temp of the radiator core in a grid.
Measure every couple inches across the top of the core.
Drop two inches and measure across again.
Repeat until you've measured the whole core.

The temp drop should be gradual. If you have any cold spots the core is plugged.