Taking 2mm off the face of a bat will most definitely have a detrimental effect on a bats performance, effectively, when we press bats, the wood compresses 3-5mm or so, so to remove potentially 2/3rds of the compressed layer will affect playability.
In terms of warranty - you'd be out of luck if it were one of mine as it's not a manufacturing fault and as others have said any self repair would void warranty rights.
With all that said however the dull sound you mention is often associated with delamination (hard wood lifting away from the soft underbelly of the bat). There are videos on YouTube on how to fix this with superglue and an iron - as the bat is already no good, and I very much doubt the shop would want to know, this may be worth attempting