It should speed up a bit as she breaks in. I wouldn't count out the 175s. Even at the lower velocities they will stay transonic farther out than those 155s.
Barrett has builder kits on their site every now and then. Happened to check last Friday and the 338/300 NM kit was in stock. Got a shipping notification on Tuesday. Should be here next week. Now I just need the 308 kit to show up on their website...