Yes, the power would be lowered if your defence was lowered. According to Serebii, "Uses the user's Defense stat in damage calculation rather than Attack stat." This basically means that in the damage calculation, the defence stat is used, so the calculation is the same except for the stat being used. As you know, normally, if attack is lowered, the damage output is less, so if defence is lowered, the damage will also be reduced. I tested this on Pokemon Showdown, and...
0 Def Blastoise Body Press vs. 0 HP / 0 Def Blastoise: 58-69 (19.3 - 23%) -- 11% chance to 4HKO
-2 0 Def Blastoise Body Press vs. 0 HP / 0 Def Blastoise: 29-35 (9.6 - 11.7%) -- possible 8HKO
I am sorry for the long response.