I studied in Brisk. what I saw 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 Bonei 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.