Hi Thomas,
I have run both 255 square and 225/255 staggered since 2009. I hesitate to make any car setup decisions until I see the BSX rules in print though. Here is my take:
All 3 setups will be pretty close in terms of lap times on most tracks.
225/255 is very fast for an experienced driver and I have logged my best TT laps with this.
255 square fits without rubbing on my car. It is fun, you can go deeper into braking zones but it really works the front brakes, is easy to rotate the car, allows 4 wheel rotation to get the most out of a set of tires, allows you to depend on the steering wheel maybe too much instead of your keen right foot. ( the steering wheel is a brake). This setup might be faster on a tight AX course but I have never set my best track laps running square. They average 1-2/10ths off but very close. It might just be the loose spacer behind the wheel.
245/40/17 square looks interesting. This might be a good compromise tire setup. 4 wheel rotation and a bit more contact patch than staggered without the extra weight and wind resistance of 255s. I may run a set next year and test them. It does still allow a novice driver to overuse the steering wheel. This will work the front brakes.