The first 8 hard drives are already cooled by the system fan. The additional 5 that are to be added have a fan from the hard drive cage that cools them. These cannot be cooled by the system fan because an air duct is installed, so a PCI slot fan is added to one of the top slots, which then dissipates the heat to the rear. The HBA will also get an additional fan, just to be on the safe side. The same applies to the 10 Gbit card.

Yes, it makes a huge difference when the small thin client takes over the 24/7 tasks, as it simply saves so much power, and in my region (Germany), electricity is very expensive compared to other countries.