Say I make an HS mule...

And use this level 81 HS mule on, say, a 5x character while training. Would I still be losing EXP due to the HS mule leeching exp, or would I end up earning more overall due to Holy Symbol?

In other words, would the level 81 HS mule leech more exp than Holy Symbol would give me?

In the old exp system, you would have lost exp. But now that the killer gets more exp for his kills, you actually get more. You will get 110%+10% (so 120%) exp for each kill.