If Finny were out of the picture, I think Gene would have been a better student but probably not as good of an athlete. Gene's natural gifts and natural drive tend toward the academic side of things.
Perhaps Gene would not have been driven to be the top student in the school if he didn't have Finny as competition, but it seems like he would have been a very good student regardless of Finny's presence.
Gene is a good student but he doesn't particularly enjoy learning. He is driven to do well, and I'd like to make the case that Gene was a good student despite Finny. Gene was constantly being distracted by Finny's antics: the Super Summer Suicide...whatever it is; the Carnival, the beach. When Finny is failing a class, Gene steps in and keeps him from sinking. I'm not a huge Gene fan; however, he is a diligent student. BUT, I'd rather have Chet in my classroom. He loves being a student and learning until the impending war changes him. Gene works at his education; Chet invests himself in his studies.
By the very fact that Gene returns to Devon School and analyzes what occurred and goes back over his past thoughts indicate that he is a very thoughtful person--perhaps, even a perfectionist. So, it seems entirely within character for Gene to always be a good student; he is clearly a student for life and of life.
To follow on from missy575´s answer, I do believe that Gene would have been a great student with or without his friendship with Finny. In fact, his friendship and the antics that Gene feels he needs to be involved him detract from his study, for example when they went to the beach and Gene goes back to flunk the first test ever. He already had his ambition and his desire to become the best student of the year, and has already developed great study habits. Perhaps this is one reason why Gene has such problems with Finny - he is already an ambitious figure who naturally would feel jealousy for somebody like Finny who is apparently so talented in one area (sports) but doesn´t have to work at it.
I do believe both competition and anger can drive someone to perform well, but I also believe in a good disciplined lifestyle. Gene indeed had strong discipline in his studies and life habits. At least the text leads us to believe that Gene was a good student prior to Finny's influence because Gene had trouble breaking the rules that Finny encouraged him to break.
A major difference between Gene and Chet is their purpose for education. Gene uses it to obtain success and power over others, it is a means to an end. Chet genuinely loves learning and therefore finds purpose in the journey. Both do proceed in an effort to be top of the class.
We all know that Finny had impacted Gene tremendously. However, even before Phineas became Gene's roommate, he had amazing study skills and was a great student. The senior year was very crucial to Gene, as it was to many other students attending the Devon School. Was Gene really patting down on his studies to prove something to Finny? Was it Gene's inner resentment toward Finny? No one knows. The bottom line is, Gene was very seriously about school, which is a very good trait for everyone to have.
However, I do think Gene would have wanted to continue to do well schollastically without Finny. I do, however, believe that Finny had a lot to do with the sudden change in Gene's studies by creating the Super Suicide Society of the Summer Session. This impacted Gene negatively because he started to drift away from his studies, but still trying to balance his work with his time with Finny.
The bottom line is, I do believe that Gene would have still become "such a good stuent" because it is his character. His charater wants to become class valedictorian and wants to get an A on everything, but Finny does put a spin on that with his character.
Chet Douglas, on the other hand really loves to learn and wants the knowledge to help him succeed in life, and Gene using his knowledge against people, pusuing admiration.