Yeah the prior person likely mispoke.  Some higher performance is possible.  ztex has boards running at 212 MH/s.  Still nobody is going to get 400 MH/s out of a Spartan so there will be no massive performance boost where a board costing roughly the same gets 25% or 33% or 50% higher performance.  The Spartan-6 is pretty well analyzed how so it is in the fine tuning.  A couple MH/s here and there.  Maybe someone gets a board going @ 220 MH/s which would be a nice 10% "free" compared to existing bitstreams but significantly higher performance is going to require a new class of chips.

There is no other 45nm chip which delivers the LUTs/$ that Spartan-6 does (without special "sweetheart" pricing) so that means waiting for the Artix-7 28nm chips to become available and at reasonable cost.  The largest Artix-7 chips likely can (eventually with a lot of bitstream analysis) achieve 500MH/s+ so a pair could provide the first GH/s board.  Still who know how long it will be (Q1-2013 ?) before Artix-7 provides better MH/$ than Spartan-6 does.