You're not the first to suggest that the corrugated pipe could be an issue. I believe the smooth radius being a positive overcomes the corrugations as a negative, just as much as the sharp angles in normal stove-pipe elbows and tees being a negative then negate the smooth walls as a positive. If I could find a mandrel bent style 8" tubing, I'd be all over it!
The best flowing option would probably be to have a flat-walled and flat roofed(kindof a square) with a swollen chamber at the elbow/turn, similar to dirt bike exhaust manifolds. Lastly, the speed of the flow has a lot to do with how much negative pressure a short section of corrugated tubing will put on the system. On a car, the exhaust flow is pretty huge. On this heater, the speed of the gases 1/2-way through the system is like a light breath exhale, so the affects of eddying and backpressure are close to zero. All that being said, at this point it is working very nicely, so I'll be leaving it that way for now.
Just had an idea; Maybe I will remove that 180' elbow and coat the entire inside of it with refractory cement, creating a smooth surface. The heat at that point is never that hot(not enough to cause massive expansion/contraction and subsequent cracking/breaking of the putty pasted into the grooves). The grooves would provide a lot of surface area for adhesion, and I could even coat the surface of the cement with a high-temp slick paint that would mimic the surface of the galvanize piping. This may be the next evolution in efficiency. :O)