A fuel pressure gauge would indicate if you had too little return flow capacity. If there was more fuel being delivered to the return line than the line could handle, it would tend to increase fuel pressure above the regulator setting.

I ended up buying the mercruiser kits to add the 2nd fuel pump and got them installed today. Going to do a test run tomorrow if the weather is good.

Been out 4 times since installing the 2nd fuel pumps and so far so good.