If my GPA and SAT are both quite a bit above average, (probs around 75th percentile) will my essays even (significantly) matter in my application? -as long as I put in decent effort
Depends, but generally, yes. What types of colleges are you applying to? You can check section C7 of a college’s Common Data Set to see how strongly they consider the essay.
The higher the selectivity and more holistic the process, the more the essays and other components like the LoR’s will matter.