Here is my argument for Roberts not being able to have a hit rate of .40 or higher. Lets list some current players who have had career hit rates close to or around .40 (with a large enough sample size obviously). Bobby Abreu, Jim Edmonds, Manny Ramirez, Jim Thome, Todd Helton (Helped by Coors), and Barry Bonds (at least since 2001 but we all know he is a different hitter since, it was around .34 before). No Albert Pujols and Alex Rodriguez dont make the list at .38 each.
What do all these players have in common? They are all big power hitters that mash the ball. The argument that Roberts speed could bring it up to that level doesnt hold any water. Though his speed will bring it up a bit, the way Ichiro has managed a .37, its not going to help it all that much.
Of course some players that aren't mashers and just other players in general have had single seasons above .40 and it is possible there is nothing to suggest that Roberts should keep up that rate for the rest of the year. His makeup and skills compare more to a hitter that will put up about .35 for the rest of the year.