Haven't really researched it but thought that you could cook sous vide for pretty much as long as you wanted as long as you set it for (and reached) the IT you wanted. If that's the case, the issue isn't 6 hours, it's the IT and how long you have the steak on how hot a grill to sear it (since searing for very long cooks the steak more). If that's true, I'd think the hottest grill (or maybe the torch) for the shortest time is the secret.
I'm gonna be really interested to see how the tri-tip comes out.