Sous Vide Frustrations

For your steaks, 5 minutes of searing seems like a pretty long time, easily enough to go from medium-rare to medium, or even more than that, depending on thickness and exactly how hot the pan is. It's definitely normal for the meat to be gray before the sear, so if your pan was indeed as hot as you can get it and you needed the whole 5 minutes to get the level of sear you want, you may want to deliberately undershoot more with the sous vide. You may also want to pat the steaks dry, if the liquid from the bag is possible taking you too long to cook off.

For the burgers, you don't mention ever salting them, which would certainly make them bland. You shouldn't mix salt into the patties, but salting just before bagging should be fine, or at least before searing. J. Kenji López-Alt suggests salting before bagging and before searing. On top of that, sirloin steak is pretty lean, which would account for the dryness and maybe some blandness. You might want to try a fattier cut, or part sirloin and part something fatty. People have all kinds of opinions about what's best, but I don't think many people like it as lean as pure sirloin.

In general, salting before bagging is totally fine and often recommended - yes, it might pull a little more liquid out, but if you want salt for flavor, it's going to be a lot more effective in the bag than added at the end. And you're going to get a lot of liquid in the bag no matter what you do. It doesn't mean the meat won't be good, although you can make a great pan sauce out of it and get even more flavor.

Poor circulation, including bags getting stuck against one side without water getting around the other side, definitely isn't ideal. It's hard to tell how big an issue it is for you, though. The best way to check would be just to sneak a thermometer probe to the places you think may not be getting good circulation and see how big a difference there is.

The pot size depends a bit on what you're making. A big stock pot is definitely big enough for a lot of things. I think the one I use is 10-12 quarts, and I do big stuff like pork shoulders. If all you're doing is a couple steaks or fish filets, you can get away with smaller.

Cascabel's answer is great, as usual. There is an important factor that I think is missing, however.

For both of the cooks you've described I would shock the meat before searing. It sounds like you are going from the bath right into a hot pan. There's no need to do this.

If you shock the meat in ice water and then optionally stash it in the refrigerator for a while you give yourself a ton of insurance against overcooking. That's certainly what is happening--your searing is adding more heat to the internal temperature of the meat than you intend.

Steak procedure

1. Process sous vide as you've described
2. Dunk the whole bag in ice water
3. Pour off the purge (bag juice) and reserve to make a kickass sauce
4. Optionally, move to the fridge for any convenient amount of time.
5. Get your pan ripping hot or get out a powerful torch.
6. Dry the surface of the steak extensively. Bone dry.
7. Season liberally.
8. Sear. A minute or two in the pan will build your crust. You can probe it if you want to make sure you don't serve it cold in the middle. Your basting process is a great way to conduct heat back into the meat.

This will result in next to no banding and you can "retherm" right up to the desired temperature.

Personally I only use sous vide to process steaks when I am really chasing that to-the-edge doneness. I generally finish on a grate over a glowing hot chimney starter.

Burger Notes

For the burger, I'm not familiar with using butter as the only fat. Typically I look for at least 20% fat in my hamburger mix. Sirloin is okay in burgers. I would go 50/50 sirloin and bacon, though. Or 25/75 sirloin to fatty chuck. I think that will resolve your blandness issue. I would expect a sirloin-only burger to be boring. All of that said, you have made this recipe before and you are happy with it so that's a bit of a non issue. Beyond that the steak advice applies here too.

Your temperatures and times seem reasonable to me. The single most confusing element is your burgers being gray straight out of the bath at 133F. That is very strange. There has to be some other variable out of whack because it just doesn't make sense. Maybe the bath overheated at some point during the cook; some kind of software issue.

How are you incorporating the butter and the sirloin? It's a stretch but are you maybe adding scalding hot butter directly to the meat and warming it from the inside?