Well.. I can say that this is definately due to ride height / compression of the rear springs.
I can still get the growling in 1st and 2nd under hard acceleration... esp. when turning right.
I have also noted over this past winter that I've gotten it with people in the back seat.
I've also hear it several times when the suspension is loaded up (compression) (driving on a rolling road... at the bottom of the trough between hills).
On all three cases.. the only common item is ride height / spring compression.
It is not soley dependent on power output... but moreso rear suspension compression.
A water pump only has one wear item.... the bearings. If you change your Antifeeze on a regular schedule (say ever 30K-35K), it should last well past 200K miles, IMHO.
I see your point, however.