Honestly the HD 800's stock cable is pretty wretched for those headphones. The conductors are small and cheap and it causes grain and hiss with a ton of recordings. A metallic haze if you will was a constant. The cable upgrade I got fixed this problem. I was thrilled cause I couldn't figure out how to get rid of that problem. Upgrading the cables for the Z 7's is less impactful no matter what quality of amps and DAC's you pair them with because they are not "the beasts of detail" that are super demanding like that of the 800's. they need quality interconnects. I found out the hard way. Lol. Anyway. Yes. I am right on this. Cables tame surface noise and harsh grain, therefore adding the cleanest background but again you gotta have the top dog headphones to notice this. MDR's are not in this level. Those stock Malaysia made cables for the Z7's are enough. No upgrade necessary unless you have some super incredible portable amp DAC like an Eddie current or something. **** those are expensive!!
I can easily hear improvements between the stock and solid silver cable that i made for my Z7 out of my the Sony pha-3. funny when u said how bad is the hd800 cable, i personally think the Z7 has even a thinner gauge wire. I agree with u though that u may hear more improvement on the hd800, but the z7 is good enough that you can appreciate the difference. the better the source and amp will allow you to hear the improvement better. Three of my audio friends also able to differentiate between my upgrade and stock cable.
i agree with jmsax69 that shielding will minimize hiss and hum from cable.