Just the opposite. Stretching helps to keep the front end down because of where the engine is in the frame. If the engine were further back then yes it would change cog some, but not enough to really do anything. If your wanting to start modding your bike for straight track the first things that come to mind is lowering the bike, especially the front and swapping sprokets. Then you just work from there. When I launch my front wheel really doesn't touch the ground until about half way through second gear but its not more than about an inch or 3 off the ground either.