I have just recently put a 700x25 tire on my rear. in order for it to not rub i have to pull the chain to a pretty tight tension.
As i am not even half way to the end of my drop-outs.. heres my question.
If i get a longer chain , could i run an even bigger tire?? and just pull out further on the dropouts.

Um. Why not? As long as your frame has the capacity for whatever size tire you want, it should work fine.

Adding a full link may end up putting your axle too far back in the droputs. A half-link may be your answer.

...

I used a half link and was able to run a 35mm tire on my IRO mark v which is stated to only have clearance for a 28mm

You could re-gear it, if you're willing to do that. Getting a chainring or rear cog with one or two less teeth will also move your rear wheel back a little bit in the dropouts.

or a $2 half link

Oh they're only two bucks? Yeah get a half link haha.