Sous Vide long cook meat prep?
Nope. Haha :)

Brining will give you a softer texture. I have brined duck for varying times (depending on how much of a rush I was in) and brining really does help with the texture.

You can sear it beforehand, some Maillard reaction will help in flavour development, but when I cook sous vide for more than 8 hours, I don't bother: the meat will go green/grey, which means I need to sear afterwards anyway. So I usually just sear at the end.

If your meat doesn't have that much fat, or anything to keep it moist, it can go dry. Adding some butter, or maybe rendered Wagyu fat in the case of your steak, might help.

As for bacteria, the longer you cook sous vide, the more you kill off the nasties.

It so happens that Douglas Baldwin has a short bit about flat iron steak: http://www.douglasbaldwin.com/sous-vide.html#Beef

Hope that helps :)