I've never dipnetted the Copper and Kenai on the same years, so I've never been able to do a side by side comparison between the fish, though we have caught and consumed 100's of them over the years. Also where the Copper fish are caught they've been in the river for over a week vs. catching them at the mouth of the Kenai. I do believe with the longer run the Copper fish make that they have a higher oil content and hence are a bit sweeter.
To me it is not so much the river as whether the fish is handled in a premium way: caught, immediately bled, gutted, put in an ice slush and delivered to market within 48 hours while having been kept on ice the whole time then yes I would say it is worth a premium as fresh ocean caught salmon cooked within a few hours to a few days of being caught is a premium product worth the effort or price.
Properly handled and cooked people who aren't even salmon fans seem to have no problems finishing their portions and asking for more. All it needs is a light seasoning and to be cooked until the flesh just firms up.
If you look at where I cut into the fillet on the right hand side you can see the internal flesh hasn't quite turned color but pulling it off at this point it finished firming up by cooking under its contained heat.
I've used sockeye as a substitue for many tuna recipes and find it makes a wonderful sashimi, poke, seared etc.