In my experience, I can definitely tell the difference between my 6 foot cables, a couple different UPOCC ones, an OFHC one from sideways, and the stockers. Big enough difference between them to easily tell (ie not a placebo). Sideways also listened to them, and was able to tell as well. 7-8 feet of a better cable would definitely make a difference.
Whether you want UPOCC or silver, or OFHC, I dunno though. Personally, I haven't tried silver for headphone cabling, as I'm a bit tentative about what I read about it being very bright. To me, that's too much extra coloartion from the cable, I'd rather just keep the sound signature the same (maybe I'm a purist). I do like a bright headphone, but I don't want to go over the top on already bright headphones (liket he 701 for example). I prefer to just clean and tighten everything up with UPOCC.
IIRC, I think sideways actually preferred his OFHC cable to the UPOCC cable, though. The UPOCC cable really tightened the bass dramatically IMO as well as improving clarity a lot overall. The OFHC improved clarity too, but not as much IMO, and seemed to have more bass than the UPOCC because (IMO) it wasn't as tight.. was a little looser so seemed to have more quantity. The UPOCC also smooths/cleans out the top end enough that they seem a little brighter, which makes the bass seem less. Basically between tighter bass reducing the quantity, and smoother/clearer/cleaner highs taking away from the bass a little, the bass on the UPOCC, compared to the OFHC, seemed to have less quantity, although it wasn't less than the stocker. I think in the end, sideways preferred the OFHC because of the bigger bass presence, but both the OFHC and UPOCC cables were a dramatic improvement over the stock cables on both the HD600 and K702 (only heard the DT880/600 with UPOCC, not OFHC).
I don't know if 38 years ago stock cables were better or worse than today, but either way, I can about guarantee you that a new (better) cable will sound much better. If I were you, I'd start with the OFHC because it's a great improvement and is cheap enough, and with the design sidways is doing for you, it will be trivial to try a different cable(s) in the future.