Not sure what you are trying to say.

Still 40nm to 28nm is 50% die size not 85%.  (28/40)^2 = 0.49.

This doesn't mean the die will be the same size.  Yields will be lower and cost higher w/ 28nm to make product price compeititive AMD will look to shrink average die (and thus get more chips per wafer).  AMD won't leave "space unused".  They make chips on giant 12" wafers.  Smaller die = more chips per wafer = lower cost per chip. 


The 7850 isn't the 6950 replacement.  It only offers roughly the same performance despite having half the die size.  So AMD is going to charge the same price as 6950 despite their cost effectively falling in half.  Have you forgotten about competition.  If AMD does that, nobody buys 78xx series chips and Nvidia laughs all the way to the bank w/ their competitively priced 28nm chips.

Chips get faster.  That is a reality in silicon products.  Yeterdays high end is tomorrows entry level.

Nothing is static.  Just because 6950 performance cost a lot and is a upper mid range card doesn't mean it will always be that.  The 7xxx specs show us AMD will be moving 6950 performance down to entry level cards.  6970/6990 level performance becomes the new mid range and 6990+ performance becomes the the high end.