You should waste a can trying it out. You shouldn't have to find out just when you've implemented your last line of defense that the HALT "spray" yields a dribble of liquified pepper that doesn't get much past your hand. Plus hitting a dog in the eyes - the only place it works - is nearly impossible even when you are stopped. The dog isn't staying still in a pose with its paws spreading its eyelids open so you can slosh some in.

You can't outrun a dog. You can only hope to get out of its territory before he / she bites you. Since the dog consumes most of your attention, you're not looking for potholes or watching out for vehicles either. Do yourself a favor and get off the bike and pick up a rock. Or, even though you're not going to use it (why blow $40 on fido), bear spray will definitely put you on top if things get really out of hand and your confidence of this will give the dog pause.