Soy sauce is a classic beef marinade ingredient along with worchestershire sauce, brown sugar, garlic and ginger.
Does the soy sauce still provide for a crust on the meat (Maillard reaction)?
See the difference is that when I add to beef MINCE you don't taste soy sauce, it just elevates everything via umami and saltiness and the result is it tastes more beefy if anything. Doesn't work nearly as well with pork and it becomes dominant with chicken/turkey mince. My Serbian wife always mixes beef and pork mince, something I don't personally enjoy.
Soy sauce in itself has undergone the Maillard reaction, I think proteins and sugars in the meat do it for burgers. In any case my burgers are crispy outside and juicy inside, I usually present them to guests as "Possibly some of the best burgers you've ever had" and many people agree. Recipe is quite simple:
1kg lean beef mince (makes about 8 generous patties)
2 eggs
2 slices of white bread lightly soaked in milk
2 tbsp of soy sauce (always bought Kikkoman)
2 tbsp Swiss mustard (nowhere near as sweet as German or American mustard...American mustard is a joke for a European, it's sweet as candy...and not at all hot like French mustard)
1 tbsp of
Vegeta
+/- a splash of sunflower oil, though I personally don't like fatty patties and usually skip the oil
The mix needs to be thoroughly kneaded to the point that you don't see meat fibers. Grill, enjoy!