A 35 degree difference between grate and lid sounds about right, which would mean the Weber included thermometer is way off. Which also sounds about right! I have the Taylor Tru Temp digital thermometer, and I'm quite happy with them. They're about $15...http://www.target.com/gp/detail.html/182-1162023-0425409?asin=B0001BFJ54&AFID=Froogle_df&LNM=|B0001BFJ54&CPNG=kitchen&ci_src=14110944&ci_sku=B0001BFJ54&ref=tgt_adv_XSG10001
I just read thru the 1 star reviews...I have 3 of this model, one of the larger version including timer. I have 1 probe that won't read below 47 F, but seems accurate above that. One probe seems to have stopped--after at least a year--and I've nicked the silicone sheath on the hole in my UDS on 2 of the probes. I contacted Taylor, and they replied rapidly, saying replacement probes are only $5. Some of these people seem out of touch with reality...these read up to 392. They won't read your oven's temp when you're baking at 450. They will certainly stop working properly if the probe tip lays on the heating element. I don't know who is washing what in a sink full of water, but I don't submerge my probes. I just scrub the ends clean. The silicone sheath has withstood the heat of my UDS and my offset, and even stood up to having the door of my offset closed upon it. Regularly. It doesn't care for sharp edges, surprisingly. (sarcasm!) And you can buy 3 or 4 for the price of a maverick. Ya don't get a remote alarm at that price. But one of the 1* reviews claimed it doesn't have an alarm. DOES TOO!
I can't hear it until I'm on top of it, but I have some problems in that department. My stepson, the bat, can hear it from a block away. (exaggeration. But they can hear it from the back porch, and sometimes from inside when the doors are closed.)
Sorry I'm so wordy today. I'm worse than usual!! Good luck with your WSM!