Is your wife looking for a higher top speed or better acceleration? If she is spinning out on the 40:11 gear combination, then by all means, install a larger chain ring, perhaps even a 44T.

A 11-51 cassette seems to be grossly unnecessary if you "don't live in an area with lots of hills," and the huge gaps between cogs in this cassette cannot be good for maintaining a steady cadence or for acceleration.