I have the same false bottom from Adventures in Homebrewing. I use it for direct fire recirculating mash. I have a thermometer set below the false bottom to monitor the temperature.
It has worked really well for me. I have be sure not to recirculate too fast or the grain will compact. Also, I have to monitor the temperature very closely. I keep my Blichmann burner set to really low.
I have a little less than 4 gallons of liquid space below the false bottom. My approach to this large amount of space, right or wrong, has been to always mash with the same amount of water regardless of the amount of grain. I fill the kettle with 8 gallons of grain and begin recirculating until I reach my strike temp. I turn off the burner and pump while I add my grain to the water and then stir and recirculate slowly until I hit my target. When I hit my target, I turn the burner back on to very low. My efficiency has not been very consistent, but over 12 batches with this system I have an average efficiency of 68% with a low of 63% and a high of 80 something.
My other experience with this mash tun is that the support structure will warp under too much weight. On 10 gallon batches, where I've used at much as 28 pound of grain, the stainless steal structure will begin to crush and the corners will bend in.
I glad to find this thread, because up until now I couldn't find a lot of other user experience with it.