Made a particular note of EGT on the 717 APU the other day whilst parked on the bay. 1 pack and electrical load 77%. Second pack on saw the EGT rise to about 83% then slowly drop to 80%.
Now what fuel flow increase that involves I don't know.
As an aside, prior to engine start one needs to confirm that APU bleed is at least 27PSI. With only 1 pack on bleed usually runs at 30 to 37 psi, whereas with two on it can be as low as 20psi per side. The FCOM specifically says to check bleed with only one pack on, so generally, I operate on the ground with just the one pack.