It's hard to say. The overall writing score depends on both task 1 and task 2. Your assumption only affects task 2, which accounts for 2/3 of the overall band. Besides, for irrelevant information included in writing task 2, you only lose points for TR (Band 3-5), but not the other 3 assessment criteria, namely CC, LR, and GRA.Therefore, technically, no one can answer your question precisely unless your full answer is presented.