"etwas ausmachen" can mean a lot of things. Here it means "to make a difference".
or https://www.duden.de/rechtschreibung/ausmachen point 6 and 7
If you leave out the word "aus", as you suggested, you get:
Ich hab‘ gehört, das macht echt was.
This is not a sentence I would use or that normally makes sense to me. Das macht was? Was macht es denn? It might be clear from context in some case, so the sentence isn't entirely wrong and can make sense in some situations, but then it has a different meaning than "... das macht echt was aus.".
You need the "aus" for it to mean "I heard this really makes a difference" or "I heard this really matters".