Related: How does Infusing work?
When performing Infusion, does it matter what the quality is of the item being disassembled for Infusion?
Per answers to the earlier question, it seems the primary thing affected by Infusion is the target item's Attack/Defense value. Other stats on the item aren't significantly changed, except to scale with the Attack/Defense.
The same answers also state that the boost given in Infusion is approximately 80% of the difference (some rounding done) between the Attack/Defense values of the two items involved. However, I'm fairly certain I've had at least a couple cases where Infusion has granted the full difference - even when the 20% I should be missing would be >1 (i.e.: any amount of expected rounding wouldn't account for this).
Could it be that using Exotics for infusion grants a bigger boost (or a chance of getting a bigger boost) than Legendary or Rare items? Is there some small difference between Legendary and Rare as well?