The MCAT is a unique thing because it is weighted almost the same as GPA in importance (slightly less), but it only takes 7 hours to complete. It requires a ton of dedicated study, but I think premeds see it as something that can make up for other deficiencies (which is definitely not always true). A good MCAT never hurts though and can only help.
A wonderful MCAT doesn’t make up for a poor interview though. I don’t know of any school that doesn’t look at applicants holistically. They try and look at the whole package and judge all of it together.
There isn’t much difference between a 515 and a 520 in my opinion. Both are over ~93rd percentile, and there is really only a few questions difference between them. On the other hand, there is a big difference lower in the scale, say a 505 vs 510 (70th percentile vs 84th percentile)