1) You should be processing the whole message queue for all types of messages in one go. When using raw input, author is peeking for "not input" event types. To me, this seems suspicious for performance. With a raw input device registered, the message queue is going to be huge.
While I don't know the underlying data structure of the message queue, if we assume an array, then the author's code will do a while loop of O(N) lookups for "not input" messages and remove them.
The correct approach is to dispatch the whole queue and ignore messages of types that you don't care about in your message handler.
2) You SHOULD be using the legacy input events for games, not raw input. The OS input massaging (ex: acceleration) is something that should remain consistent between applications. If there's acceleration: that's what users expect. You don't want the mouse to suddenly behave different in your application. Your games tuning values (sensitivity, accel) should apply relative to the OS values.