Please forgive my ignorance, but the answer to this question could be of great importance to my GTA IV experience. I know what the immediate rewards are of various 100% achievements in the currently-being-played-game, but are there any benefits with attaining those achievements with regard to subsequent playthroughs, i.e., if I start a new game with only a 100% file in the saves directory, will the Buzzard, speacial weapons, and infinite-ammo-carrying capacity¹ be available to me sooner than if I were playing for the first time? If thse perks are available only in the current game, not in any new games started after achieving 100%, then the perks are absolutely meaningless. You've got all these wonderful things now, but the game is frigging OVER, so what's the bloody POINT? The benefits are of no value whatsoever unless the benefits persist- so do they? I've started my second playthrough after 100%, but I'm not far enough along to have enough money, etc, to tell. So far, it looks like the benefits do NOT persist.
¹I noticed, much to my chagrin, that this is not precisely true: You don't have infinite ammo, not exactly: You can carry/buy infinite amounts of ammo, but if you save and reload the save after you bought, for instance, 300 RPG rounds, you now only have eight rounds- 292 rounds have vanished; you only have the huge amounts of ammo you've accumulated until you either are arrested, or save (I haven't tested if the excess is there after you die, whether or not you save).