How big is a jump from tier to tier? How much % should you expect in increase if you say move up 2 tiers ex.= 670 gtx < 690 gtx
I assume you are referring to tom's latest chart:http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
For the most part a jump of two tiers is not that significant.
If you are upgrading a graphics card, make it a big jump of more than two or three tiers.
If you don't, you may be disappointed if you don't see a big improvement.
But, in the one example you gave of a GTX670 to a GTX690, it is a massive jump.
A GTX690 is about the equivalent of GTX680 sli.