That's about right.
With a CPU running to throw 80 Watts in the loop, assuming a 1 gpm flow rate (60 gph), the temp difference between the CPU block's inlet and outlet will be ~0.3 deg C.
As my buddy Martin put it
here, it's calculated by:
P*t/(C*m)=DT
P: power (watts)
t: time (second)
C: 4190 J/(kg*K)
m: coolant mass (kg)
And that doesn't account for a 2% secondary heat loss.