Dinan axle-back exhaust owner here.
During cold startup, I'd estimate that the Dinan is 25% louder than stock. After the cold startup cycle is over and the car is idling smoothly, the Dinan is MAYBE 10% louder than stock. What you notice more than the change in volume is the change in tone. The Dinan is much deeper and has a more aggressive rumble.
As far as drone, every exhaust I've ever heard for the M3, including stock, has it. It's simply a question of (1) how much and (2) where it occurs in the rev range. It's also a subjective issue that depends on the ear and tolerance of a particular person, which is why one person will say an exhaust drones and another says it doesn't.
Like the stock exhaust, the drone on the Dinan is barely even noticeable (my wife, for example, didn't notice it) and not annoying. Also, like the stock exhaust, the drone on the Dinan does not occur at cruising speeds. In fact, the drone on the Dinan is so unobtrusive, I'd have no question recommending the Dinan even to those with the most sensitive ears (I'm one of them).