I don't know about Mango (v2), but I own the MT300N v1 which is the previous version of Mango, very similar CPU architecture, and I also have the Buffalo WCR-1166DS (same CPU as Mango), you can check out this Wireguard test DB, the speed is slow, and since the CPU load was very high occasionally it just hangs, so I don't really recommend it to be VPN gateway.