Matt, I have a new 18.5 WSM. I use a Brinkman charcoal bowl instead of the larger Weber water pan that came with the unit. The Brinkman charcoal bowl has more clearance...alot more! I didn't actually measure the clearance but it appears to be the same as on the old 18.5 WSM.
Good suggestion about the adjustment but I've already taken it back to the place I bought it. They swapped it out for a Tel-Tru and didn't even ask any questions. That's what I call excellent customer service!
I'm in Kansas City, essentially sea level. All vents wide open. Ambient temp is 70 F. Currently registering 200 F.
I had one of the old WSM's with a Tel-Tru and it always ran in the 230-250 range. Wish I had my old unit back.
After the cook later this evening I will run a test with boiling...
First cook on my new 18.5 WSM and I can't get temp above 200 F with full ring of lit K. I think it's proably the temperature gage. Anyone else have problems with the new Weber gages?