So R-Value refers to the material's ability to resist heat flux and depends on the temperature difference between either side of the assembly. The general formula is Heat Loss = (surface area x temperature difference) / R-Value
So as you rightly hypothesize, the "first" bit of insulation is more important than the next bit. Going from R-1 to R-2 decreases energy loss by 50%, going from R-2 to R-3 decreases it by a further 30%, R-3 to R-4 by a further 20%, etc.
So for my example with a 40ºF difference between inside and outside, if that wall assembly were 120sq ft, the original wall with R-6.15 would be losing 780btu/hr. The 13% improvement in R-Value from the first example would reduce energy costs by 10% (700btu/hr of energy loss), the 35% improvement would reduce costs by 25% (578btu/hr). It's strictly better to increase r-value but the highest leverage impacts come from the bottom.
So as you rightly hypothesize, the "first" bit of insulation is more important than the next bit. Going from R-1 to R-2 decreases energy loss by 50%, going from R-2 to R-3 decreases it by a further 30%, R-3 to R-4 by a further 20%, etc.
So for my example with a 40ºF difference between inside and outside, if that wall assembly were 120sq ft, the original wall with R-6.15 would be losing 780btu/hr. The 13% improvement in R-Value from the first example would reduce energy costs by 10% (700btu/hr of energy loss), the 35% improvement would reduce costs by 25% (578btu/hr). It's strictly better to increase r-value but the highest leverage impacts come from the bottom.