It is with the idea of something like the "applicant cap" that I disagree generally, and so the issue of an assistantship is an instance of that. I believe that thinking of one's application in this way distorts the important fact that the writing sample is not just "by far the most important part of the application," as you put it; for applicants who were not undergraduates at an Ivy, the importance of the writing sample so dwarfs the other aspects of an application that it is simply a waste of time to worry about the other parts of one's "applicant cap." In connection with this, it is misleading to talk about high GRE scores and grades as "beneficial" to one's application. They are only beneficial in the sense that, if you do not have them, your application will likely not be considered long enough for the writing sample to be read at all. They should be considered minimum requirements, not "bonuses" or extra feathers in one's cap. The only thing they will do is persuade an admissions committee to take a close look at your writing sample, and since an assistantship is far less important than GRE scores and grades, you should expect that it will do very little work in getting you admitted.