I'm two hours into my cook with the foiled clay pot mod. This is a great mod.. I'm cooking ribs and pork loin filets. The ambient temp is 35 degrees, it's windy, with some snow flurries. I have a double charcoal grate and the charcoal ring is half full. As Dino indicates, you have to control temps on the way up but this is not hard to do with the wsm. Right now two vents are opened about 1/2- 1/4, one is closed. The lid temp is 235-245. So far everything looks great. Under these weather conditions, the clay pot seems to be the right solution. Once the ambient temps rise to the 80's or 90's this summer, I will probably return to water to keep temps low. The clay pot I'm using is stamped number 36 made in Italy, 14" diameter (purchaed from Home Depot for approx. $5.00). The second grate is sitting on top of the clay pot and is approximately 1/4 inch above the straps. The water pan is well foiled, and the clay pot is well foiled. This possibly accounts for grate not touching the straps, although I will have to verify this for sure when my cook is done. However, the grate resting on the pot is still sturdy and I don't think it will pose any problems on the cook or on convection. Granted, I have nothing cooking on the second grate at this moment but I can't see how a cook would be impacted.
With regards to moisture, I just took off the pork loin fillets and they were still moist. I don't see any difference in cooks with water.
In summary, the clay pot mod. is defintely the right choice for colder weather. Mike
Update: Had a successful rib cook, lasted for 5 hours. WSM was very responsive to the vent settings. Felt like I was adjusting the dial on an oven. I don't get this type of responsiveness with water, as it is continuously being evaporated. In the last two hours of my rib cook, I opened the vents 3/4 to full, and easily attained 250 to 275 at the lid. When the cook was over and WSM was cool, I checked for fuel consumption. To my amazement, I had used approximately 1/4 of the charcoal ring full of charcoal.
The clay pot was very hot to the touch while I was taking the top layer of foil off with the drippings right after my cook. The clay pot kept the temps very consistent. Very impressed. Mike