From a process perspective, I don�t know if you should be concerned ... You've told us nothing about your process.
THE BOOK
JEDEC MO-51 allows 6 thou warp in smaller [1mm pitch] BGA and 8 thou in larger [ 1.27mm and 1.5mm pitch] BGA. The increase Motorola laid on you was allowed by JEDEC because the manufacturers had a tough time meeting the 6 thou requirement in the larger packages.
PRACTICE
As long as you maintain symmetry of materials and balanced copper, warp should not be an issue. Generally, balanced product has warp so low that it is essentially unmeasureable.
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