Drops are determined by item level. Stats are affected only in the way that certain affixes only spawn above a certain item level - the same item will, however, always have the same item level no matter where it dropped.
Tal Rasha's (as any other high tier set) has an item level of 61. That means it can spawn with most, if not all, of the highest tier affixes. It also means it can drop anywhere in Inferno, from an Act 1 barrel to Diablo herself. Where it drops will have no bearing whatsoever on what stats it can have.
By the way, here are the item levels for all the Acts of Inferno:
Act 1: 61
Act 2: 62
Act 3: 63
Act 4: 63
As for items... for now, you'll have to look up items manually on the Blizzard page ( http://us.battle.net/d3/en/item/ ) if you want to find out their item levels. In a future patch, this will be displayed in the tooltip.
05/06/2012 12:55Posted by IDKLOLDoes Act 4 and/or Whimsyshire have a higher chance to drop higher level items than Act 3?
12/06/2012 17:01Posted by DoriotI dont know about Whimsyshire, but I do know that Act 4 on Inferno difficulty dropps ilvl 63 twice as often as it does in Act 3.
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