Some devices are more sensitive to temperature variations than others: whether you send 90 C water or 80 C water to a radiator, the difference in performance will be small - maybe 10%. But if you send 0 C water instead of 20 C water to a heat pump (in heating mode), you will utterly destroy your efficiency. Refrigeration cycles require very specific operating temperatures to run efficiently - fuel-fired heating systems are a completely different animal. Many heat pumps list performance at different temperatures - have a look at how fast the output changes with it.
Still, it surprises me that you would design a boiler plant for a 20 C temperature rise. They are typically rated for much less for safety reasons: I've seen boilers melt their burners due to insufficient water flow and you also don't want to accidentally boil the water.
A gas furnace, on the other hand (if that's what you are talking about) is designed to operate at an extremely high temperature (stainless steel burner), so it can handle a very wide range of temperatures: you might see a 40 C delta-T for the air going through a gas furnace. As long as you keep the output temp below about 60 C (so you don't burn anyone), you can do just about whatever you want with it.