You are right. Smart Array P822 controller is supported on a DL380 G7, but the P812 which is supported, doesn't do SATA 3.0 (6 Gbps), which is what the 4 SSDs in the system use. This is a homelab system, so there isn't any budget to support buying SAS SSDs, or HDDs for that matter, to get 6 Gbps speeds.
If I enable No-Battery Write Cache, the controller can use the cache, and throughput (to the drives) increases. My question is whether the controller utilizes the connected capacitor, in case of unexpexted power loss, while that option is enabled.