i created a ticket at rockstar support not for the jaggies but for the low res textures und blurry ground textures stuff.
i got a long answer from the support team.
posting now the only interesting part:
2.) Clear the Game Cache.
Clearing the Game Cache on the Xbox 360 is a separate function from clearing the System Cache. In order to do so, you must hold the LEFT Bumper, RIGHT Bumper, and the X button all at the same time on the Xbox 360 controller as a game is launching. Once you reach the Main Menu or Title Screen of the game, you can let go of the buttons. There will be no notification to you that this process has completed because it is all done passively.
Clearing the Game Cache resolves many problems in games that rely heavily on the Xbox 360's cached memory. Some of these problems include, but are not limited to NPC issues, random textures, and "hitching", "lagging", or "slow gameplay".