I changed my counter sprocket from the stock 13 to a 12, I am also thinking of changing the rear from the stock 40 to a 42, has anyone done this, Is it a noticable difference?

 

Yes, it is very noticeable. I don't know why you'd want that much low end though. 12/42 is a hair above an 11 tooth front sprocket. A good rule of thumb is 3 teeth on the rear = 1 tooth on the front.

 

What kind of riding are you planning on doing? This would be(to me anyway) the main determining factor. I recently at the Atoka test and tune, ran a 14/40 combo, then switched to the 12/40 combo, and really (by the time slips) could see very much difference at all. Yes, it would pic up faster, but with the 14 I was able to stretch each gear out a little further. I think the stock 13 needs to be thrown as far and hard as you can...Try both sprockets the 12 and the 14...I like the 14 my self(even still good for the trails) ......JMO