Charlotte and I tested Transistor damage rolls in a link battle. My Cresselia's HP is 381 and its Special Defense is 303 (Calm, no EVs), and it is not holding an item.
Our rolls:
142, 147, 130, 142, 132, 130, 138, 126, 141, 127, 126, 142, 144, 126
(lowest roll 126, highest roll 147)
The damage calculator's rolls:
252 SpA Transistor Regieleki Thunderbolt vs. 0 HP / 0+ SpD Cresselia: 144-169 (37.7 - 44.3%) -- guaranteed 3HKO
Possible damage amounts: (144, 145, 147, 148, 150, 151, 153, 154, 157, 159, 160, 162, 163, 165, 166, 169)
We think this is 1.3x, but we're DEFINITELY certain it's not 1.5x any more.
I also don't own a Regidrago.
TheMantyke is cool and good for testing Dragon's Maw.