Now I'm confused, so correct me if I'm wrong: Cable that comes with N8 is good enough to charge N8 as fast as the phone circuitry allows it. Will having better cable increase speed of downloading/uploading data and if so how much?
It depends on several variables as I understand it.
Snapdragon (Supports QC 3.0) vs. Exynos (Adaptive Fast Charging Only)
Quick Charge 3.0 (QC 3.0) is faster than Adaptive Fast Charging
The Cable that is included with the Note 8 is designed for Adaptive Fast Charging and the Charger is Adaptive Fast
Charging as well. (The factory cable should work just fine with QC 3.0 I haven't confirmed this, but I assume it would)
Aftermarket QC 3.0 charger and USB 3.0 or greater cable will give you a faster charging rate than the Factory Charger/Cable on Snapdragon Chip based hardware only.
Other factors that affect charging rates are cable length and quality as someone else pointed out earlier in this thread.
Don't confuse charging rate with data rate. If all you're concerned about is fast charging rates, don't spend the extra money on the USB 3.1 Gen 2 cables, just get the USB 3.0 (aka USB 3.1 Gen 1). They both have the same max voltage output.