No, there is NO RWHP increase by changing rear gears. This is exactly the same as saying "my car has more HP in first gear than in second gear"
This is NOT the case.
If you put you car on a dyno, and run it in first you will get a certain HP reading. Then run it in second you will get the SAME reading.
Now put your car on the dyno with a 2.8 rear gear and take your HP reading, then swap the rear gear for a 3.75, your HP will be exactly the same and at exactly the same engine revs. It will show max HP at a lower road speed but it will be the same figure.
(there is potentially slightly different friction between different rear gears, but this would be so small as to be ignored)