The 8th Principle of faith listed in the siddur is:
אני מאמין באמונה שלימה שכל התורה המצויה עתה בידינו היא הנתונה למשה רבינו עליו השלום.
I believe with a complete faith that the entire Torah that we have now was given to Moshe Rabbeinu.
That is hard to swallow. The gemara already says that we are not experts in חסרות and יתרות so it could be that in our Torah there is an extra or missing vav or yud and thus different from the Torah given to Moshe. Already Chazal related that there were 3 Sifrei Torah in the Beis Hamikdash and there were slight discrepancies between two and the other one and the Rabbis ruled to follow the majority. So they were just following the majority as per halachic practice but that doesn't guarantee that the two were correct.
It is well known that until this day our Sifrei Torah differ slightly from that of the Yemenites. Rav Ovadiah in the sixth volume of Yechave Daas has a teshuva [I believe נ"ו] where he discusses this issue and the halachic import. Rav Yaakov Kaminetzky in Emes Li-yaakov [p. 388] even suggests that in Vayikra [20/23] where we have the word הגוי the Rambam's text read הגוים. So if we believe that our text exactly was given to Moshe then according to the nusach of the Ani Maamin in the siddur we would all be kofrim because the Rambam had a slightly different version. The Shaagas Aryeh even writes in a teshuva that since we don't know the precise text, writing a sefer Torah is only a mitzva dirabbanan b'zam ha-zeh.
The bottom line is that it would seem we need not believe that our text is the exact one to the letter given to Moshe. What we MUST believe is that Torah was given to Moshe at Sinai. In addition he writes that we must also believe that the Oral Law was handed down to Moshe. This is omitted by the author of the Ani Maamin in the siddur.
So the belief in Torah Mi-sinai is binding. Not to believe is to lose one's portion in [this world and] the next world.The belief in Torah Shebial Peh is ALSO binding and critical. The finer details of various letters [and the last 8 psukim - see the end of the first perek of Bava Bara] is beyond the scope of our knowledge. And who knows - maybe when Moshiach comes we will find out that our text IS exactly identical to the original. [See Yeshurun 22 p.704 and the sefer Da'as Emunah Vol. 2 where he quotes a lengthy teshuva from the Rashba on this topic]. The problem is that the Rambam himself writes in the Peirush Hamishnayos -
והיסוד השמיני, הוא התורה מן השמים. והוא שנאמין כי כל התורה הזאת המצויה בידינו היום הזה, היא הנתונה על ידי משה, שהיא כולה מפי הגבורה.
Like it says in the siddur. וצ"ע