A theory that i had was that he was pretending to be loyal but was
actually plotting to overthrow the government. Would this be a
plausible storyline considering what happens in the book?
I suppose anything is possible, but that is not really at all how he is written. I think we are supposed to take the character at face value. In any case, he only appears in one scene (in the cafeteria, as I recall), so I don't know if one can read too much into it. Also, the kind of things that he says during that scene don't exactly suggest a secret conspirator.
Then again, maybe he is the best actor in Airstrip One.
The scene starts with
'Just the man I was looking for,' said a voice at Winston's back.
In an intellectual way, Syme was venomously orthodox. He would talk
with a disagreeable gloating satisfaction of helicopter raids on enemy
villages, and trials and confessions of thought-criminals, the
executions in the cellars of the Ministry of Love. Talking to him was
largely a matter of getting him away from such subjects and entangling
him, if possible, in the technicalities of Newspeak, on which he was
authoritative and interesting.
'It was a good hanging,' said Syme reminiscently. 'I think it spoils
it when they tie their feet together. I like to see them kicking. And
above all, at the end, the tongue sticking right out, and blue--a
quite bright blue. That's the detail that appeals to me.'
From reading, he seems as if he is conniving and calculating.
I must say, that is not how he comes across at all to me. Conniving with what aim in mind? I think Orwell put him in there as an example of an intellectual who was owned by the party. This is a fairly common phenomenon in real life, and Chomsky for example has some good things to say about it.
However, he does disappear in the course of the book, so I suppose you could read something into that if you so chose.
A handy reference is the AU Gutenberg version: http://gutenberg.net.au/ebooks01/0100021.txt