can I use the plural form of 'efficiency' when talking about multiple devices?
Certainly, I'd say, though perhaps not in the context of your example, where I prefer your less-favoured option: "Reducing the supply temperature of the heating system increases the efficiency of a heat pump". The cases match, and since this statement seems always to be true, I don't see the need to suggest multiple efficiencies.
("Reducing the supply temperature of the heating system increases the efficiency of heat pumps" seems to have mismatched cases, even though I know you could have multiple heat pumps per heating system.)
In using a plural there is the possibility of interpretation of each heat pump having several efficiencies (for example, as determined in various operating conditions)
This I believe is the main reason why "efficiencies", "powers", "throughputs" and so on are useful in engineering writing.
The examples LPH gives ("…studied the efficiency of mutual fund companies") work fine, but I can imagine a situation where "the efficiency" would wrongly imply that the figure is constant across different situations.