its a good idea to knock 10-15% off the Mh numbers on that wiki page.  most are from extreme overclocks.

i have 4 mildly overclocked 6870  cards and all i can get out of them is 270 or so. 

i am running the cards at 940 (stock is 900) core and the memory is not adjustable as far as i can tell, i can set it to 300 but at 100% load it shoots up to 1050 or something.

if i push my cards to 980 they crash, these are very well ventilated cards in an open 2 level case made from aluminum with with about 3" between the cards on the bottom level and 3" between the cards on the top level pulling cool air in one window and exhausting hot air out another window.  i have never been able to get them even close to 300Mh