I'm trying a new combo, using these mons:
Drifblim @ Sitrus Berry
Stockpile, Minimize, Baton Pass
Ninjask @ Focus Sash
Protect, Swords Dance, and Baton Pass
And finally into
Aggron @ Aggronite
Brick Break, Rock Slide, Earthquake, and Heavy Slam
So my Mega Aggron has a base Speed stat of 102 at level 50, which means that after +6 Speed Boost, the speed would be at 408.
So how would Unburden affect that? Would it double the stat to a stupid speed of 816? Or would it raise it by 2 stages, making it 510?