Reb Nachman's statements in this regard are not well known because they are hinted at in his published books but explicit in the השמטות on the חיי מוהר''ן which was only printed once a long time ago by the Na Nach group.. However I have heard that that actual השמטות were collected [by Rav Shmuel Horvitz] and are available in print in the bookstore on Rechov Salant in Mea Shearim.
I should mention that the חרם signed by the Gra --if you actually take the time to read it, did not apply to Reb Nachman which is why I feel free to read his books. But in terms of what the Gra did actually forbid I believe is forbidden and remains forbidden because of the same problems that Reb Nachman saw. To ignore the signature of the Gra is to invite insanity. And every time one ignores it, he or she invites another drop of insanity into his or her soul.
[The Ari is quite an amazing thinker and I think it was in part by learning his books that when i got to Israel the Divine Presence started shining on me and my family. Eventually I pushed that away since I did not feel the ability to remain attached to God on that level. It was just a bit too much for my poor soul.]
[In terms of learning the Ari I feel besides his basic works there are a few schools of thought later on which are fine commentaries of the Ari like the books of Rav Yaakov Abuchatzeira, and the Rashash (Shalom Sharabi) and the Ramchal.]