hmm, are you saying that the regenerator (or recuperator) has lower efficiency at lower ΔT? or lower efficiency when it's more a ΔH than a ΔT? because if that 5° of cooling and dehumidification costs you the same US$100 a month as the 30° of heating, saving 80% of it will still be US$80 a month
Using a regenerator to get 5°C of cooling back with some 30% of efficiency probably won't be cost-effectice. But if you have to heat by some 30°C, the numbers change a lot.