Although I am not an ESL teacher, I have a little experience in administrating placement testing. In my opinion, you should keep the old test questions because I am pretty sure you get more students whose ability and skills are not at the highest level, and who simply will not be able to provide any writing sample if you raise that bar. However, you should develop more academic essay questions for students who scored really high on the test (from what I understood, you see the test results before you give out the writing portion of the test?). This way you will not overwhelm lower level students and will get a better picture of where the higher level students stand.
By the way, what happens if the student scores really high (95 to 100 if you are using "1 to 100" scale)? Does he still have to go through ESL program or can he "jump into" a college English classroom?