Actually, Aluminum fins would increase performance over copper. Copper is a great solid to solid conductor of heat, but Aluminum is far superior solid to air when it comes to heat, so having a Copper tube to Aluminum fin would work great.
The catch here is the connection point between the materials; such as with an air cooling block, the fins are not soldered, they are physically 1 piece so the heat transferer efficiency is 100%, where as if you were to have a Cu block and Al fins soldered on, you would lose efficiency and counter act any gains from Al cooling fins. A radiator is a different animal, it cant be 1 piece, so Cu tube to Cu fins soldered on, vs Cu tube to Al fins soldered on the transfer efficiency is the same and Al takes the lead.
What surprises me is the EKWB, I always thought of them as the gold-standard and its clearly labeled on their website that tubes are CU H90. So for this to be brass, not only does it go against my perception of the company, this is a lawsuit (myself included as I have a Quantum P-series radiator.) Granted I havent had any issues and it works good, I never even paid attention to the details in the spec sheets, it is very annoying to read this.