I own 2 Browning SS rifles - but these are the original A-Bolts - not the Newer A-Bolt IIs. I have heard, but not confirmed, that the original A-Bolts are a better design than the newer A-Bolt IIs.
As for the BOSS, I have a Winchester Classic SS in 300 Win. Mag with the BOSS. Does it work? YES! Just make sure that you don't move the adjustments after you set them, because sometimes just a little turn in the BOSS can make a big difference in accuracy.
However, keep this in mind. My barrel with BOSS is 26" - not bad for a magnum rifle. However, the BOSS is actually about 2 1/2" long. As such, the true length of the barrel is only about 23 1/2" long. Still not too bad for a magnum, but not as beneficial as a 26 incher.
Now keep in mind that the WSMs in the Brownings are 23" long. That means that the true barrel length is only about 20 1/2" long. In my opinion, that's just too short for a magnum. I would harbour a wild guess that a regular .270 with a 24" barrel will have almost the same velocity as a 270 WSM with a 22.5" barrel - that kinda negates the benefit of the WSM.
As such, would I get the 270WSM in the browning WITH the BOSS? Well, that's a two fold question. As for the first question, I don't think that I would buy a new browning SS. As for the second question, I definately wouldn't buy it with the BOSS because of the shorter barrel.
Zachary