User blog comment:B214/Question and Answers/@comment-50.11.177.49-20130708052828/@comment-5185939-20130716234610

Juggernaut Drive was supposed to grant the wielder the full power of Albion and Ddraig, but in a berserker state.

The fact that both both allow them to surpass the power of Juggernaut Drive suggests one of two things.

First, that it grants the same level of power as Juggernaut Drive, but with more control, and fewer limitations.

Second, that the Sacred Gears somehow let them tap into more power than the Heavenly Dragons wielded.

Now, with Issei, that could easily be the combined power of the eight Pawns promoted to Queen, but still doesn't explain how Vali could surpass Juggernaut Drive.