yea... I bet the chimney works a lot better now that it's not obstructed by chunks of sheet metal ...
Although, if you want extra height, why not take a 4-6' piece of casing or sch 40 or whatever and attach it t the top of the existing casing using either clamps, welding, or heavy sheet straps.
I do have a 2 ft pc of casing just debating how to make a flange i have a welder but leads are not long enough to reach that high. i was thinkin a slip or somthing to the nature, but i dont want the damn thing comin down n conkin me on the noodle
when I wack the chimney with a hammer, the way I clean it is give it a couple wacks, u hear the soot/cresote fall, then I open my cover at bottom and hoe it out, alot easier than gettin up on the roof on a bad day, I guess its just the price you pay for being thrifty, might try a strap deal, but what would you suggest? I thought of maybe goin to a heavy truck parts store and seeing if i could possibly get a exhaust band clamp that would close down enough to hold it, but I dont know how much one of those would be and size, it is 6 3/8 round so I imagine that an 8" band should work???