Value of upgrading only 1 set of interconnects?

Answers will vary however I've experienced success with upgrading one pair of interconnects. If one has a good ear for music and an overall understanding of mixing and matching great success can be achieved. It's tunning the music to your preferences. With that said many people will suggest using the same brand interconnects etc.
It's all good!

BTW: which Cardas balanced cables are you currently using?

I've had good results changing just one pair. I fared better with keeping the best pair near the source.