After investigating, the following were my findings:
"As part of the NetApp qualification process, there is no performance degradation with the qualified X90-412B-R6 / SP-412B-R6 10K RPM drive as the seek distances are reduced to be equal to a full stroke 15K RPM drive."
As for the physical size difference:
"The only difference is that the 412A is a 3.5 inch HDD while the 412B is a 2.5 inch HDD. The difference in size is negated by a bracket within the carrier assembly in the 412B. So while the two HDD’s look different, the 412B is fully compatible with the 412A."