By coincidence I just did this with a hunk of sirloin. My butcher called it Chateaubriand but they were liberally using the classic not modern definition, the sirloin closest to the filet, tougher but more flavorful. Some customers should feel hoodwinked, but this was exactly what I wanted.
We made a marinade of bruised rosemary, smashed garlic, and olive oil. Next time we'd add some lemon zest. We warmed the packet sous vide to 138 F for 2 hours, then briefly grilled at high heat to establish the exterior texture and flavor.
Using some fat in a sous vide packet is an updated version of the classic French "confit". One can reproduce duck confit this way; the only difference is the fat doesn't taste rancid. Non-traditionalists prefer this modern twist on a classic.
The effect was like a great steak, almost tender like stew or prime rib. There are those who theorize that more than an hour is too much, because the meat goes to mush. With tougher cuts one wants this effect, at least a bit. Be aware of the issue, and tune each cut separately to taste.
138 F is also a matter of taste. This was medium rare, but unquestionably cooked through. Plenty of people would pick 134 F, some would go even to 140 F.