I studied in Brisk. what i percieved from my rosh yeshiva was that they use regular ashkenazi siddurim but incorporate their own nuschaos (ie. most often following the gr"a, for example, ommitting  **נא**  in the blessing of rachem in bentching. also **ברחמיו** in boney yerushalayim. However they don't follow every addition/subtraction of the Gr"a for example they do not add a blessing for modim derabanan or add nachem in bentching on shabbos