When draining the wort from the tun...

into the boil pot, after batch sparging, while using a March 809 pump to do so, is there any point to closing the spigot 1/2 way or should I just open it up and let it free flow? Will it improve efficiency by draining slower?

Running it full open can compress the grain bed and potentially cause a stuck sparge.

..or so I'm told.

I don't use a march pump, but after vorlauf, I always run full open. I use rice hulls every time, and don't ever get a stuck sparge. I have had a stuck sparge when not using rice hulls, though. FWIW I get about 80% efficiency in an igloo 10gal MLT w/ steel braid.

From everything I read, you should not restrict the flow on the input side of a march pump. I would suggest throttling at the output side. I'm assuming you have a ball valve on the output side of your pump.

My first brew after getting my pump, I opened everything wide open, compacted my grain bed, and got a stuck spage. IT was a bit of a pain, but not as bad since I had a pump. The two brews since I have opened the tun valve all the way, closed down the output side of the pump, and had perfect flow, crystal clear wort, and nostuck sparge.

Steve, how much did you restrict the flow on your pump? 30% 50%? 70%?

A decent amount. I would probably say 50%. I might also add that recirculating the wort via the pump for two minutes prior to draining has given me 8%, 12%, and 14% increases in efficiency in my 3 batches with the pump.

I can't say what the *correct* amount is, but for me
wide open = stuck sparge
@50% restriction = beautiful wort.
YMMV

It would also depend on what your separation medium is. False bottoms, at least in my experience, are the most prone to a stuck runoff due to compaction. I close the pump output valve by at least 30% during the runoff and then slow it even more right towards the end to give the wort enough time to fill the deadspace.

It takes a while for the liquid to drain through the grain bed. If you drain to fast you'll drain out most of the liquid. If you dig a small hole through the grain to look at the bottom of your tun it will look like you drained everything. However, if if you then wait a few minutes and check, there will probably be a couple inches of wort that has percolated down. Depending on the design of your pickup tube, it can be difficult to pump this out now that you don't have the pressure to start the flow again. Better to just throttle it back a little to allow the liquid the percolate down.

That being said, I do two sparges after first doing a run off. I typically run my pump at about 50% when I first drain and for the first sparge, but then throttle it back to less the 25 % for the final sparge. With a pump the vorlauf is so easy I have no problem with doing a second sparge.