I've did the same with my gen 6 Legacy, with the same results. A very slight delay and then eye popping full on max brakes, screeching to a stop. I would not want to do this at highway speeds.At speeds above 30mph, the rear tires would lock up and the car would likely start yawing into a skid.
I've tested engaging the e-brake while under way in a parking lot at low speeds. It was pretty dramatic even then. The system has no modulation. It just puts the brakes fully on when activated.
It's possible the G5 and G6 have more developed systems, but I've never seen any mention of it.