Do keep in mind that having 300ns L3 latencies comes with it's own set of problems. Even at 10Gbit 300ns is not enough to get a packet through an electrical connection. Plus they also have some 90ns latency products.

That means that these switches, while fast, cannot check the packets for correctness (they don't have the full packet). That they will have "aborted" packets. That in some important ways these networks have the problems of the "half-duplex" networks of old.

Broadcom focuses on features for packet transmission. That means these Mellanox switches are pretty much restricted to situations where you want to have a set of servers on a single network segment and nothing else (not even an upstream connection). If that's exactly what you need, great. But mostly you're going to need more.