RE the RB25, Ignoring the obvious capacity differences...
A much larger bore will give it more torque, along with a way better flowing head...it goes with out saying.
The redtop 20 also appears by reports to flow better than the silvertop, along with slightly more aggressive cams...
My biggest regret is going for the front facing plenum, standard is more than adequate for 99% of the RB20's out there.
I really dont see anyone making more power out of cam gears in a 20, more so a little fatter midrange, and not a huge difference in response, perhaps on a snappy 2860 it would be a different story.