I go with 2.5x based on Rild's statement.
Episode 19
Time: Roughly 19m40s
Context: Rild senses Goku's ki after Goku transforms into a Super Saiyan
Rild: “Up to now, you haven't been putting out half of your power.”
Take this for what you will, but imo it makes sense to be anywhere in between 2-3x.
SSJ2 I have at a measly 1.25x boost. When I'm feelng dicey I'll make it 1.5x. Goku literally used it to nullify one of Rild's attacks, that was all it was useful for. He didn't think it would change the state of the battle by staying in the form. Or it is simply more draining in GT for :toei reasons.