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I have the smaller 1988 cast iron manifold,
and including pipe, my total exhaust weight is 12 pounds. What appeals to
me most is that it is compact, simple, and I don't have to worry about thin
walled pipes glowing red inside my cowling, as the cast iron manifold
absorbs a lot of heat that would otherwise turn exhaust pipes red hot.
The noise reduction with thick headers is real. Thin headers on cars make a noticeable increase in the noise level. They make a lot of high-pitched "ping-hiss" sort of noise. (At the instant you first disconnect an air tool, it makes this sort of sound.) It is like you can hear the valves opening through the thin-wall header pipes. The pipe itself also rings with each exhaust pulse.
A thick-walled pipe only delays the temperature rise and spreads out any hot spots. It does not reduce the final, steady-state, average surface temperature. The thin-wall pipe will indeed have more prominent hot spots. These hot spots will appear on the outside radius of the bends as they leave the engine, where you can see them easily.
It is probably be a good idea to use thick-walled pipe for the first several inches, and then transition to thin-walled pipe. I don't know, but I suspect that the stock manifold is quite a bit thicker than it needs to be, however.
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