It definitely feels powdercoated but either way why would the electrical conductivity matter here? Am I missing something obvious?
You are missing the whole ATX standart and how electronics are designed in mind for consumer use.
Basically those brackets are conductive and look like that for a reason.
It is the shortest path to the ground. If for some reason an artist thinks he's smarter than engineers doing their job for decades then these things can happen.
1. Static Discharge. Events when you connect peripherals cables, touch the device, you introduce the device to few kV of charge that should have been dissipated via shortest ground path, if you cut it off it will travel to signal ground and PCB traces aren't that far away and that kV charge will overlap in to data lines.
Outcome. if you are lucky the device will just hang, the other is a dead device. In factory in the final product phases there are fun phase, where you take a taser and zap the device ports to test if the grounding is done properly so the customer doesn't damage the device. If you take away that grounding plate, you brake the defensive countermeasures for that.
2. EMI. There are two coins for it. So you render the device susceptible to outer EMI traveling via cables, shielding, as the shortest path again for the noise to travel will be via power ground though the card, pcie slot and motherboard. Depending on your connected devices and surrounding you compromise stability of the whole system, it may cause increased coil whine, freezes and other weird events, like monitor dropping signal etc.
The other event is that the card itself is a 0.5kW transmitter, if you cut off a trace that should have been grounded, no return path, you basically make an antenna, thus causing other devices to malfunction, let it be on your motherboard or outside in your room.
Outcome.
Well if my dear colleague in RMA service is generous, and doesn't void you warranty just because you changed the heatsink, he will write to Taiwan and tell you either way that the block killed your precious 2000$ investment. They will most probably ask to enquire Alphacool why they have designed a product not complying with ATX rules. Art before functionality answer won't cut here, it is 2K, you want accept any risks? Ok if you have warranty, but what after that?
You can ask old Bill if I am wrong here, he's a good RF guy. But it applies with the condition I asked before, if the bracket is conductive. Powder coated usually are isolated and that's a desired feature, like for socket brackets Thermal Grizzly makes, so it doesn't short the traces, but not here.