The passage says "shall be saved" and "be saved." These are in the future tense, which to me would indicate that they lead into something, likely baptism. They do not say, "are saved" and "have just been saved." I could walk around saying anything I want, but if I don't do anything about it, what good would it do?
I'm not so sure that I agree with the setup of your argument. The passage also starts with "if thou shalt", so the whole thought is framed in the future tense.
As far as the OP, I don't see any reason that this passage should be a point of contention for Orthodox and Protestants. I also don't see how it is any sort of proof for or against instantaneous salvation. I think "salvation" becomes problematic when it is decided that it is one thing. Is it instantaneous or is it a long process of theosis? Perhaps that is an incorrect question to ask. I think what really needs to be asked is, What does it mean to believe and confess? I think scripture and Tradition show us that there is more to believing and confessing than the "Believer's Prayer". Salvation does not exclude that first moment of conversion either.
My 2 cents...