My understanding of meat cookery is that meat has very little flavour until it is browned over high heat. That is why you brown the meat before casseroling, nothing to do with "sealing the flavour in", it's to create the flavour through the Maillard reactions on the meat surface. I use a very high heat and get beef and lamb very brown before adding liquid.
With mince it's all surface and I cook it with a little olive oil over high heat until it looks, frankly, much too brown (takes 15 to 20 minutes) - but of course, onceit is cooked into the sauce, everything blends together, and the meat plumps up. Delicious