The power specifications of the Hypex class D amplifier modules into low impedance loads are limited by thermal capacity, not maximum current. Both the NC500 and NC1200 do have current limiters, but they are set at 26A and 38A respectively, so the NC1200 does have the ability to deliver more current into very low impedance loads. However, even 26A is enough to provide around 950 watts into a 2ohm load.
Most of the modest priced Hypex based designs use the same power supply for the NC500 and NC1200 based designs. I think this is bigger limitation than the maximum current capabilities of the modules themselves. It's possible that the added current capabilities of the NC1200 might have some sound quality benefits even if you are not invoking the current limiter. I have owned a Nord NC500 MkII, but have not heard the NC1200 so can't comment directly on the sound differences.
I think the bigger issue is whether you would benefit from the added thermal capabilities of the NC1200. Most of the class D amps on the market that use these Hypex modules have only modest heat sinking. In most cases, they modules are bolted directly to the bottom plate of the amp which doesn't provide much in the way of extra heat dissipation. While class D amps are very efficient, they aren't 100% efficient. As the speaker load impedance drops, the amp's efficiency drops since the current increases which must be dissipated by the output transistors based on their on-resistance, and during switching.
The Hypex amp's efficiency is about 95% into 8ohms, but only about 88% into 2 ohms. So if you are putting 1000 watts into the speaker at 2ohms, the amp has to dissipate 120 watts. This is a huge amount for the small plates the amp modules are mounted on and the thin amplifier base plate. Consider the size of the heatsinks in the FirstWatt amplifiers, which dissipate about the same power (a bit more for some models). So high power demands can quickly push the amplifier into thermal overload.
Fortunately, when we are listening to real music, the vast majority of the time, the amp is only generated a tiny amount of power. I don't listen super loud and my speakers are a bit more efficient (92db/w), and when I'm listening, my amp is putting out less than a watt most of the time. I have to really crank them to get peaks over 100 watts (based on my measurements).
Bottom line, both of these amps have exceptionally low output impedance (and thus very high damping factor). I doubt you will notice a significant difference in sound quality unless you are exceeding the amps thermal capabilities. These modules do incorporate some soft-clipping features to try to prevent overload when the amp is being pushed. So if you are really pushing it hard, this would have an effect on sound quality.
