...can the CSP really make that much of a saving ?
Yes.
Most normally aspirated Lycomings can be run 'oversquare' by up to 4 'units'. eg. 30"/2,600 rpm (often seen at the start of the takeof run on high pressure days), or 26"/2,200 in the cruise. This is considerably more fuel efficient than running 'square' settings for the same power/same airspeed.
MJ
Ps.
"2300/65%. 2400/75%. 2500/80% 2700/100%.
I assume this is benchmarked to sea level at ISA
And to a particular fixed pitch prop as well. I've never noticed this plate, but I'll look more closely at the next opportunity!
MJ