Frequency response of system with feedback and feedforward co-efficients?
Yes, you can always use the FFT to compute the frequency response of a discrete-time system at discrete frequencies. For an IIR filter you get the following transfer function:

$$H(z)=\frac{\sum_{m=0}^{M}b_mz^{-m}}{\sum_{n=0}^{N}a_nz^{-n}}\tag{1}$$

Usually, $a_0$ is normalized to 1. In order to evaluate $H(z)$ on the unit circle $z=e^{j\theta}$ you can compute the FFTs of the numerator and of the denominator of (1) separately and then divide to get the frequency response of the filter.

If you use Matlab or Octave, you can simply use the command $\tt{freqz}$.