Back on Saturday, before this pinched nerve in my neck started screwing with my left arm, I tried hooking up the Facet 7 psi thumper pump again on the MB, now that I have the fuel pressure gauge installed on it.
Basically, the results were underwhelming. The Facet pump is a drop in the bucket compared to the mechanical supply pump on the MB.
Not really surprised. Based on the MB FSM, which speced 300cc/min at 150 rpm cranking out of that pump, if the calculations hold up that beast should be flowing like 95 gph at 3000 rpm.

I'd like my fuel tank shaken AND stirred please!
From what I've seen of the flow rate coming out of the return line at idle, when running diesel purge thru it from a liter plastic bottle, I could well believe it can achieve that flow rate.
Hooking up the Facet pump so that it was pushing fuel thru a Permacool filter, the diesel fuel thermostat/coolant heater, and then to the suction of the mechanical pump, running the Facet pump with the engine shut off only registered 1 psi on the fuel pressure gauge.
With the engine running, it didn't make enough of a difference to even be detectable on the pressure gauge - it was drowned out by the fluctuations from the mechanical pump.
There did seem to be a side benefit from running the Facet pump by itself with the engine shut off - it apparently flowed enough fuel at even 1 psi to either flush all the air out of the system upstream of the mechanical pump and/or flush out some of the rust and dirt that may have gotten past the filters from a couple recent tanks of cruddy fuel.
The next day when I drove it, with the Facet pump removed, there was a definite difference in noise level - the clack was still there, but was definitely reduced in loudness and harshness, and went away at a lower rpm. The effect gradually went away after a few stops - air accumulating again in the filter and thermostat upstream of the mechanical pump?