R’ Moshe’s psak applies to the entire Brooklyn, including Boro Park. My Rebbe was vehemently opposed to it, and said that anyone ho argues on R’ Moshe regarding it has no business being a Rav.
That said, I do have to share a discussion I had with him, because R’ Hershel Shachter does say differently from R’ Moshe on a matter. He said the following:
Regarding the Brooklyn eruv, there are people who say R’ Moshe was wrong, and they disagree with his psak. With regard to R’ Shachter’s psak, he didn’t say that. He felt that the circumstances had changed, and that R’ Moshe’s psak didn’t apply anymore. He said that were the circumstances the same, he would definitely agree with R’ Moshe. That is permissible. I later heard from a friend of mine who is a close talmud of R’ Shachter that before R’ Shachter gave the psak, he spent 2 sleepless nights reviewing it, and he was literally shaking at the thought of issuing a psak that differed from what R” Moshe had said – and this even when he agreed with the psak, but felt the circumstances had changed!
I guess my point is that it really depends on the reasoning of the psak. If a Rav feels that times have changed, and the psak no longer applies, he can give a psak. I just don’t understand how a posek in NY can argue on R’ Moshe without having a comparable posek to base his psak on.