Polygons pours under BGAs?
I would not put a copper pour on the top layer of the PCB directly under a BGA.  Not because of any signal integrity issue, but because of a manufacturability issue.  There are two main reasons:  


The increased copper could (and I emphasize the "could") suck too much heat away from the pads causing bad solder joints.
Normally the soldermask is pulled back a small amount from the pads.  If your copper plane had a direct connect to the pads (which it should) then the effective area for the pad would be increased.  And the increased pad size could cause solder issues where the amount of solder required for this ball is more than the solder required for a ball not connected to the plane.  The "unevenness" of the solder per ball could cause solder issues.


I'll be the first to admit that both issues could be solved by changing the oven temperature profile, changing the solder mask pullback, and/or changing the size or thickness of the solder paste screen.  But why?  Unless you have a very specific requirement (RF maybe), I don't see the advantage.  I've lost track of how many multi-GHz BGA's I've used and I have never needed a plane on the top layer of the PCB.