I've been working on filling up my fly boxes for next year. I've never tried tying a comparadun before but, this year I'm going to try. You're right the thread body gives it a nice slim profile just like you would see in an adult mayfly. Most of my mayfly imitations are tied parachute style or Klinkhammer style which works up here but, I think those flies would be drowned in the turbulent streams of the Smokies
Deerhair is hollow so even a comparadun should float well in the streams in the Smokies. I've never heard of using an old fly line for a body before. It would certainly be a fun experiment to try. When you use a lighter to taper the body do you need to it in well ventilated room? Would there be an issue with the fumes given off?