8th order butterworth frequency response
Confused - something must be wrong in your calculation - at least for the 3rd stage.
According to the Qp values as given in the TI document (I gave you the reference earlier in another thread) the 3rd stage (second order) of an 8th order Butterworth filter must have a value Qp=0.9. As I have mentioned earlier, each stage with Qp>0.7071 shows a gain peaking at the pole frequency (for Butterworth identical to the 3dB cutoff frequency).  However, in your figure the 3rd stage has no peaking and, thus, a pole Q&lt;0.7071.

EDIT: More than that, the peaking of the last stage seems to be too large. 
How many dB? For a Q-factor Qp=0.9 the peaking is only app. 1 dB.