[greglepore]

I had a Breville for a long while (a Dual Boiler) and it was very sensitive to puck height. They actually have a little tool that you can use to "cut" the top of the puck to exactly the right headspace. Too low and the shots run fast, really too low and they're soupy. I'd try 18g to start with the normal double basket, but you want it to be as full as it can be and still allow the portafilter to lock in place.
The preinfusion on the Breville's is adjustable. Go for something longish, like 7 sec or so. Aim for a puck that's firm enough to get a 35 sec shot at a minimum with that preinfusion. Make sure the beans are freshish, like no more than 20-30 days out of roast. Report back.