The answer is in this topic: Titans: Grades, Loot Tiers, and You
But I’ll give you the TLDR:
A+ rank on a 5* titan = Loot Tier 8
C rank on an 8* titan = Loot Tier 8
So as long as you maintain C rank – which is at least 1% of the Titan’s HP in damage – you will get the exact same loot tier as you would getting A+ rank (1st place) on a 5* titan.