These are some calculations done for this:
Regular:
0 Atk Rattata Tackle vs. 0 HP / 0 Def Pikachu: 55-66 (26 - 31.2%) -- guaranteed 4HKO
(55, 55, 57, 57, 58, 58, 60, 60, 60, 61, 61, 63, 63, 64, 64, 66)
With lowered Defense:
0 Atk Rattata Tackle vs. -1 0 HP / 0 Def Pikachu: 84-99 (39.8 - 46.9%) -- guaranteed 3HKO
(84, 84, 85, 87, 87, 88, 90, 90, 91, 93, 93, 94, 96, 96, 97, 99)
With increased Attack:
+1 0 Atk Rattata Tackle vs. 0 HP / 0 Def Pikachu: 84-99 (39.8 - 46.9%) -- guaranteed 3HKO
(84, 84, 85, 87, 87, 88, 90, 90, 91, 93, 93, 94, 96, 96, 97, 99)
So yes, boosted Attack is the same as lowered Defense