I don't know about the V600, but he V700 carriers came with little "feet tabs" that adjust carrier height to one of three settings (low = no tabs, high = tabs in one direction, medium = tabs turned around).
When I first used my V700 I noticed it's sharpness was not nearly as good as my old dedicated slide scanner. I attributed this to film carrier height.
To discover the perfect height I scanned with all three settings and compared. I noticed that the medium and high settings looked about the same, which led me to believe that the ideal setting was somewhere between the two. So I stuck a business card under the feet of tabs in the medium setting, and the sharpness was better. I then added a few layers of masking tape to the bottom of the medium tabs to match the business card thickness.
If I'm scanning badly curled film I make sure to check the sharpness and adjust the tabs to whatever works best.
The V600 carriers do not have that. I'll play around with it this week and see what I can come up with... I never thought to judge sharpness by the grain - I was always just looking at the edges of stuff.
Using the grain to focus is just a habit I adopted from making prints--focus aids in print making are actually called "grain focusers." The grain is independent from other factors that might affect focus, like depth of field and diffraction.
You probably can't lower your carrier, but you could experiment with raising it a little bit--just cut out some paper to sit it on, and see if focus improves.