1. 열심히 공부한 만큼 성적이 잘 나오겠어요. (민큼 is short for 만큼으로(adv) = 만큼(n: the same amount) + 으로(by))
2. 열심히 공부했으니만큼 성적이 잘 나오겠어요. (만큼 is a particle adding the sense of "as much as" to -ㅆ으니(까))
The two have about the same meaning, but #2 indicates the cause and effect more clearly.
-ㄴ 만큼 and -ㅆ으니 만큼 can mean different things in other contexts.
1. 시험 성적은 공부한 만큼 나온다 = Your test score will reflect how much you have studied.
2. 시험 성적은 공부했으니만큼 나온다 (X) Wrong.
-ㄴ 만큼 in (1) means "as much as" or "directly tied to the amount", indicating a general relationship.
It can also be used to mean an actual occurrence. 나오겠어요 in your example refers to what's about to happen in reality.
-ㅆ으니만큼 on the other hand is based on -했으니(까), which means "as you have ...ed", indicating what actually happened. -만큼 only adds the sense of "exactly as much" to -ㅆ으니. Thus it can only be used about what actually has transpired and not for a general rule, and its meaning is clearer than -ㄴ 만큼 with no possibility of misunderstanding.