Is this a deadly Martial Art Combo?

Sure it's a good combo, but you're better off learning one well than learning two halfway. If you're already at a high level at BJJ then go for it. Your body will benefit from the different type of training and boxing is the striking art in the U.S. Im not saying others aren't great as well, but most boxers train to actually fight, so it is a different mindset and there is no BS.