The grammar portions [of the SAT writing test are far more important [than the essay]. And they're very easy to prepare for. In fact, there are only a handful of grammar rules that the SAT loves to test, and they repeat them over and over. So if you can just memorize the rules that we'll lay out in this section, you'll find that the actual test won't be that much of a challenge. During test prep, scores on the writing section tend to increase most reliably, and by the most number of points.I believe it.
Hack the SAT by Eliot Schrefer
Last week I skimmed an SAT roster for our high school from a couple of years back. I saw no 800s on math, one 800 on reading, and four 800s on writing. (Not sure how many students in that group -- perhaps 125?)
I think that's consistent with Schrefer's observation that the writing section is easiest to coach.