OK I tried it on my desktop PC with ASUS supremefx audio (ESS DAC + amp) with low and high gain (up to 600 ohms), no difference. With my HiFiman HE400s the high gain does make an impact (rich and faster bass), but with my experience with IEMs the extra power really doesn't do much at least for my ears. So
@1clearhead you might indeed have a dud.
As for the silver/gold upgrade, to my ears the gold cable sounds the same as the stock cable. It looks nicer though and the wire feels better (the stock is a slightly sticky rubber) and softer, great option if you don't need the mic.
Usually on taobao the sellers sell the upgrade cable bundle cheaper than getting the upgrade cable separately, if that's the case I would suggest to do it since 1. it's hard to find 3rd party cables for KZ, as the pins are standard 0.75mm but the housing is proprietary and 2. it's pretty cheap anyway and always nice to have a back up cable. Also if you decide to remove the metal memory wire like me, I have some concerns on the wire relief near the earbud side, so best to have a spare cable just in case.
As with the ZS3 and ZST, getting the upgrade cable option should mean you get BOTH the stock and upgrade cables. Especially with the ZS5 where the stock cable is inside the box with the earbuds, while the upgrade cable is in a separate box.
Also, blue ZS5 + gold just looks fantastic...I just wish the upgrade cables had mics as well which I do use frequently...