Assuming fully invested Pikachu in both stats with a neutral nature each
Its attack equivalent to an un-item boosted base stat of 160, regular stat of 419 (at lvl. 100), sp.atk equivalent to un item-boosted base stat of 149, 397 at lvl. 100.
Its attack equivalent to a life orb boosted stat is a base stat of 111, regular stat of 321, sp. atk equivalent to life orbed base stat of 103, regular of 305.
Its attack equivalent to a choice banded boosted stat is a base stat of 90, regular stat of 279, sp. atk equivalent to choice specced base stat of 83, regular of 265
Method (in case this is where I screwed up):
I punched in Hyper Beam and Giga Impact on the Calc (actually the one at gamut-was-taken.github.io, my parents block Showdown) using 252 Atk/252 SpA Pikachu holding a life orb against a 0/0 Manaphy not holding any item. I saw that 252 Atk Light Ball Giga Impact did a min of 191 damage (in raw HP) and that 252 SpA Light Ball Hyper Beam did a min of 181 damage. Thus, I kept modifying the base stats of Pikachu changing the items as necessary.