Pull your Wagyu at 185° internal or it will be overcooked.
With all the marbling I think it will cook faster than the average brisket.
FTC (Foil, Towel, Cooler) should keep it hot enough you'll have trouble holding it with bare hands after 4 hours.
From Paradise Lockers:
There are a few differences when smoking wagyu briskets compared to angus or hereford briskets.
1. The wagyu will cook faster. Heat will transfer faster through fat than through meat and with the intense marbling that the wagyu carries it will warm faster than your standard briskets. You will have to use your own judgement as far as cooking time goes but you can use this as a base to start - at 220 degrees a 11 lb brisket will take approximately 6-9 hrs to smoke and cook fully, depending on the outside temperature and weather.
2. You will not finish the wagyu at the same temperature that you do the standard briskets. For home cooking, we recommend pulling the wagyu somewhere between 150 and 160 degrees internal temperature, wrapping and letting it rest for half to a full hour before slicing. For competition cooking, we recommend pulling the brisket between 170 and 180 degrees internal temperature and follow the same resting procedure as above. The extra ten to twenty degrees will give you a little additional breakdown and allow the slice to pass the judges "pull test". The fat content on the wagyu has a lower melting temperature than other beef so taking these to 190-200 degrees will actually cook out all of the fat and you will not get the rich, elegant taste that wagyu is known for.
3. The use of injections and marinades for added flavor and tenderizing with the wagyu is NOT a necessary step in preparation. The intense marbling gives the wagyu a rich flavor and tendrness that in our opinion, needs no other flavors added. Rubs are still good for the outside of the brisket to add a flavor profile to seperate yours from everyone else.