100% copper? That is a first. I truly hope it works well for you. It doesn't exist, but then 12 AWG of ATMOSPHERE inside a PVC tube
should be an eye opener.
Do you have any idea how many types of copper there are? I don't. I'll guess and say there are a FEW different types at LEAST.
I guess YOU use the types that uses LARGE letter and in color.
As for copper extension cable, they work as well as any off the spool PVC/Vinyl .999 OFC copper. The color may vary, other than that,
OFC is good copper for most application. It's usually SC, stranded and both can be tinned.
The tube/dielectric and size is the deal breaker.
In most AV and HT applications the fact that zero return happens at a fairly low point in cables, makes a choices pretty simple.
The same is true for picky equipment, go ahead use extension cord (cable). You're the one that has to listen to it, not me.
For movies and HT "who cares", seems to be the general consensus from quite a few, go with that. Whatever makes your pickle squirt.
Birds of a feather, after all.
Regards