Definitely trim the hard fat off the packer. Also trim up some of the fat layer that separates the point from the flat. Aaron Franklin has a good video on Youtube on a channel called BBQ with Franklin. It will take you through how to pick out a brisket, trim a brisket, and cut a brisket.
There is nothing wrong with cooking at 275 to cut your cook times down. There is also nothing wrong with ramping up the temperature towards the end to reduce cook times.
My personal preference for doing briskets.
Trim fat cap of hard fat and about 1/4in thick around.
Rub with 1 part kosher salt, 1 part course ground pepper, 1/4 part garlic powder (by volume)
Throw in uds or kettle running at 300-325 fat cap down.
At 4ish hours check for color, if it is at the color you want wrap in foil or butcher paper. If not at the color you want let it keep riding.
At the 5 hour mark start probing the thickest part of the flat for tenderness
If not tender, check every 20-30mins until it probes tender.
Pull off smoker, vent heat off for 20mins, then wrap tight again and place in an oven (that is off) for 2 hours
Slice and enjoy.
I prefer my brisket with pecan wood, but oak is good on it as well. Oak and Cherry mixed together on brisket is very dericious as well. I woudn't smoke with mesquite, as it is very aggressive. I use mesquite to throw onto the coals when I'm grilling burgers/steaks etc.
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